ENH Display error messages in admin with an admin context

[GuySartorelli]

Currently if there is an error in any /admin route, there is no admin-specific way of handling the errors.
This means that if silverstripe/errorpage is installed the user is shown the frontend error page (if there is one) and if it isn't installed or doesn't have a page for the given error, a simple text message is displayed.

This PR renders an error message in the context of the admin, so that users can simply select an appropriate section in the left menu.

The error detail only appears in Dev mode.

In this example I tried to navigate to /admin/badonk and an appropriate 404 error is displayed

In this example I tried to navigate to /admin/pages/save and an appropriate forbidden error is displayed

Parent issues

• fixes No error handling for non-existient CMS section #1232
• fixes No error handling for missing controller action silverstripe-cms#2669

Depends on

• ENH Let LeftAndMain handle its own errors. silverstripe-errorpage#67

[emteknetnz]

Much nicer having error messages in the CMS then going to the front-end

A few things

[emteknetnz]

It's not really a page that does not exist - it's a route that does not exist. Maybe a "section" is better language?

e.g. /admin/security/asdfasdfsadf is will say this page does not exist, though you wouldn't call /admin/security a page

[GuySartorelli]

True. I think "section" has the same problem as it may be for example a specific ID in an edit form that has no associated data object.
How about something like "You were trying to access a section or object that doesn't exist"? Or simply "The URL you were trying to access resulted in a 'Not Found' error"?

[emteknetnz]

@GuySartorelli looks like this has caused regressions - https://github.com/silverstripe/silverstripe-cms/runs/7571102762?check_suite_focus=true

1) SilverStripe\CMS\Tests\Controllers\CMSMainTest::testCreationOfTopLevelPage Error: Typed property SilverStripe\Admin\LeftAndMain::$httpErrorMessage must not be accessed before initialization

[GuySartorelli]

#1331 fixes that.