[ESUI] More robust handling of error responses (#96819)

* more robust handling of error responses

* added tests and further hardening of how we handle error values

src/plugins/es_ui_shared/__packages_do_not_import__/errors/handle_es_error.test.ts

@@ -0,0 +1,71 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+import { errors } from '@elastic/elasticsearch';
+// eslint-disable-next-line @kbn/eslint/no-restricted-paths
+import { kibanaResponseFactory as response } from 'src/core/server';
+import { handleEsError } from './handle_es_error';
+
+const { ResponseError } = errors;
+
+const anyObject: any = {};
+
+describe('handleEsError', () => {
+  test('top-level reason is an empty string', () => {
+    const emptyReasonError = new ResponseError({
+      warnings: [],
+      meta: anyObject,
+      body: {
+        error: {
+          root_cause: [],
+          type: 'search_phase_execution_exception',
+          reason: '', // Empty reason
+          phase: 'fetch',
+          grouped: true,
+          failed_shards: [],
+          caused_by: {
+            type: 'too_many_buckets_exception',
+            reason: 'This is the nested reason',
+            max_buckets: 100,
+          },
+        },
+      },
+      statusCode: 503,
+      headers: {},
+    });
+
+    const { payload, status } = handleEsError({ error: emptyReasonError, response });
+
+    expect(payload.message).toEqual('This is the nested reason');
+    expect(status).toBe(503);
+  });
+
+  test('empty error', () => {
+    const { payload, status } = handleEsError({
+      error: new ResponseError({
+        body: {},
+        statusCode: 400,
+        headers: {},
+        meta: anyObject,
+        warnings: [],
+      }),
+      response,
+    });
+
+    expect(payload).toEqual({
+      attributes: { causes: undefined, error: undefined },
+      message: 'Response Error',
+    });
+
+    expect(status).toBe(400);
+  });
+
+  test('unknown object', () => {
+    expect(() => handleEsError({ error: anyObject, response })).toThrow();
+  });
+});

src/plugins/es_ui_shared/__packages_do_not_import__/errors/handle_es_error.ts

@@ -38,12 +38,14 @@ export const handleEsError = ({
     return response.customError({
       statusCode,
       body: {
-        message: body.error?.reason ?? error.message ?? 'Unknown error',
+        message:
+          // We use || instead of ?? as the switch here because reason could be an empty string
+          body?.error?.reason || body?.error?.caused_by?.reason || error.message || 'Unknown error',
         attributes: {
           // The full original ES error object
-          error: body.error,
+          error: body?.error,
           // We assume that this is an ES error object with a nested caused by chain if we can see the "caused_by" field at the top-level
-          causes: body.error?.caused_by ? getEsCause(body.error) : undefined,
+          causes: body?.error?.caused_by ? getEsCause(body.error) : undefined,
         },
       },
     });