Elasticsearch: Fix handling of errors when testing data source (#28498)

#24999 changed the error response payload, but the test data source
was not adapted to this change and broke the feature of displaying
any errors to the user in the UI. This change should resolve this
problem.

Ref #24999
Ref #28481

public/app/plugins/datasource/elasticsearch/datasource.ts

@@ -338,12 +338,8 @@ export class ElasticDatasource extends DataSourceApi<ElasticsearchQuery, Elastic
       },
       (err: any) => {
         console.error(err);
-        if (err.data && err.data.error) {
-          let message = angular.toJson(err.data.error);
-          if (err.data.error.reason) {
-            message = err.data.error.reason;
-          }
-          return { status: 'error', message: message };
+        if (err.message) {
+          return { status: 'error', message: err.message };
         } else {
           return { status: 'error', message: err.status };
         }