ENH Display error messages in admin with an admin context

_config/forms.yml

@@ -16,3 +16,7 @@ SilverStripe\Forms\Form:
 SilverStripe\Forms\FormField:
   extensions:
   - SilverStripe\Forms\FormMessageBootstrapExtension
+
+SilverStripe\Control\RequestHandler:
+  extensions:
+    - SilverStripe\Admin\AdminErrorExtension

code/AdminErrorExtension.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace SilverStripe\Admin;
+
+use SilverStripe\Control\Controller;
+use SilverStripe\Control\Director;
+use SilverStripe\Control\HTTPRequest;
+use SilverStripe\Core\Extension;
+
+class AdminErrorExtension extends Extension
+{
+    /**
+     * Used by {@see RequestHandler::httpError}
+     */
+    public function onBeforeHTTPError($statusCode, HTTPRequest $request, $errorMessage = null)
+    {
+        $controller = $this->getAdminController();
+        if (!$controller || Director::is_ajax($request)) {
+            return;
+        }
+        $controller->setHttpErrorMessage($errorMessage);
+    }
+
+    private function getAdminController(): ?Controller
+    {
+        if ($this->owner instanceof LeftAndMain) {
+            return $this->owner;
+        }
+        if (Controller::has_curr() && (Controller::curr() instanceof LeftAndMain)) {
+            return Controller::curr();
+        }
+        return null;
+    }
+}

code/LeftAndMain.php

@@ -175,6 +175,11 @@ class LeftAndMain extends Controller implements PermissionProvider
      */
     protected $pageID = null;
 
+    /**
+     * Set by {@link LeftAndMainErrorExtension} if an http error occurs
+     */
+    private string $httpErrorMessage;
+
     /**
      * Assign themes to use for cms
      *
@@ -773,6 +778,32 @@ protected function init()
         Versioned::set_default_reading_mode(Versioned::get_reading_mode());
     }
 
+    public function afterHandleRequest()
+    {
+        if ($this->response->isError()) {
+            $this->init();
+            $errorCode = $this->response->getStatusCode();
+            $errorType = $this->response->getStatusDescription();
+            $defaultMessage = _t(
+                self::class . '.ErrorMessage',
+                'Sorry, it seems there was a {errorcode} error.',
+                ['errorcode' => $errorCode]
+            );
+            $this->response = HTTPResponse::create($this->render([
+                'Title' => $this->getApplicationName() . ' - ' . $errorType,
+                'Content' => $this->renderWith($this->getTemplatesWithSuffix('_Error'), [
+                    'ErrorCode' => $errorCode,
+                    'ErrorType' => $errorType,
+                    'Message' => DBField::create_field(
+                        'HTMLFragment',
+                        _t(self::class . '.ErrorMessage' . $errorCode, $defaultMessage)
+                    ),
+                ]),
+            ]), $errorCode, $errorType);
+        }
+        parent::afterHandleRequest();
+    }
+
     public function handleRequest(HTTPRequest $request)
     {
         try {
@@ -1998,4 +2029,23 @@ public function getVersionProvider()
     {
         return $this->versionProvider;
     }
+
+    /**
+     * Get the HTTP error message if one has occurred during HandleRequest.
+     */
+    public function getHttpErrorMessage(): string
+    {
+        return $this->httpErrorMessage;
+    }
+
+    /**
+     * Set the HTTP error message when one occurs during HandleRequest.
+     * Called by {@link LeftAndMainErrorExtension}
+     * @internal
+     */
+    public function setHttpErrorMessage(string $message): self
+    {
+        $this->httpErrorMessage = $message;
+        return $this;
+    }
 }

lang/en.yml

@@ -33,6 +33,12 @@ en:
     CollapsePanel: 'Collapse panel'
     DELETED: Deleted.
     DropdownBatchActionsDefault: 'Choose an action...'
+    ErrorDetail: 'Error detail: {detail}'
+    ErrorMessage: 'Sorry, it seems there was a {errorcode} error.'
+    ErrorMessage403: 'Sorry, it seems the action you were trying to perform is forbidden.'
+    ErrorMessage404: 'Sorry, it seems you were trying to access a section or object that doesn''t exist.'
+    ErrorMessage500: 'Sorry, it seems there was an internal server error.'
+    ErrorMessage503: 'Sorry, it seems the service is temporarily unavailable.'
     ExpandPanel: 'Expand panel'
     HelpMenu: 'Help menu'
     LOGOUT: 'Log out'

templates/SilverStripe/Admin/Includes/LeftAndMain_Error.ss

@@ -0,0 +1,24 @@
+<div class="cms-content flexbox-area-grow $BaseCSSClasses" data-layout-type="border" data-pjax-fragment="Content">
+
+    <div class="cms-content-header north">
+        <div class="cms-content-header-info vertical-align-items flexbox-area-grow">
+            <div class="breadcrumbs-wrapper">
+                <span class="cms-panel-link crumb last">
+                    $ErrorType
+                </span>
+            </div>
+        </div>
+    </div>
+
+    $Tools
+
+    <div class="panel panel--padded">
+        <p>$Message</p>
+        <% if $isDev && $HttpErrorMessage %>
+            <p>
+                <strong><%t SilverStripe\Admin\LeftAndMain.ErrorDetail "Error detail: {detail}" detail=$HttpErrorMessage %></strong>
+            </p>
+        <% end_if %>
+    </div>
+
+</div>

tests/behat/features/notfound.feature

@@ -0,0 +1,39 @@
+@gsat
+Feature: Not found
+  As a site owner
+  I want error messages to be displayed in the context of the admin section
+
+  Background:
+    Given I am logged in with "ADMIN" permissions
+
+  Scenario: Errors are displayed in the admin context
+    Given I go to "/admin/nothing"
+    Then I should see "Not Found"
+    And I should see "Sorry, it seems you were trying to access a section or object that doesn't exist."
+    And I should see the admin menu
+    Given I go to "/admin/pages/nothing"
+    Then I should see "Not Found"
+    And I should see "Sorry, it seems you were trying to access a section or object that doesn't exist."
+    And I should see the admin menu
+    Given I go to "/admin/security/EditForm/nothing"
+    Then I should see "Not Found"
+    And I should see "Sorry, it seems you were trying to access a section or object that doesn't exist."
+    And I should see the admin menu
+    Given I go to "/admin/security/EditForm/field/Members/nothing"
+    Then I should see "Not Found"
+    And I should see "Sorry, it seems you were trying to access a section or object that doesn't exist."
+    And I should see the admin menu
+    Given I go to "/admin/settings/nothing"
+    Then I should see "Not Found"
+    And I should see "Sorry, it seems you were trying to access a section or object that doesn't exist."
+    And I should see the admin menu
+
+  Scenario: Valid routes do not display the error
+    Given I go to "/admin/settings"
+    Then I should not see "Not Found"
+    And I should not see "Sorry, it seems you were trying to access a section or object that doesn't exist."
+    And I should see the admin menu
+    Given I go to "/admin/security/EditForm/field/Members/item/new"
+    Then I should not see "Not Found"
+    And I should not see "Sorry, it seems you were trying to access a section or object that doesn't exist."
+    And I should see the admin menu

tests/behat/src/AdminContext.php

@@ -37,6 +37,24 @@ public function iShouldSeeAnInvalidTabIcon(string $not, string $tabLabel)
         }
     }
 
+    /**
+     * @Then /^I should (not |)see the admin menu/
+     * @param string $not
+     * @param string $tabLabel
+     */
+    public function iShouldSeeTheAdminMenu(string $not)
+    {
+        $selector = '#cms-menu';
+        $page = $this->getMainContext()->getSession()->getPage();
+        if ($not) {
+            $element = $page->find('css', $selector);
+            Assert::assertNull($element, 'The admin menu is visible when it should not be');
+        } else {
+            $element = $page->find('css', $selector);
+            Assert::assertNotNull($element, 'The admin menu was not found');
+        }
+    }
+
     /**
      * @When /^I can (not |)see the form validation error message$/
      * @param $not