Display top-level error reason if no shard info is available

For EQL queries, error responses do not contain `failed_shards`
information, and so our error toasts contained only a stack trace.

With this addition, we'll fall back to the top-level `error.reason` if
those fields are not present, giving the user better indication of
what's going on without having to inspect the actual network response.

src/plugins/data/public/search/errors/es_error.tsx

@@ -22,7 +22,7 @@ import { EuiCodeBlock, EuiSpacer } from '@elastic/eui';
 import { ApplicationStart } from 'kibana/public';
 import { KbnError } from '../../../../kibana_utils/common';
 import { IEsError } from './types';
-import { getRootCause } from './utils';
+import { getRootCause, getTopLevelCause } from './utils';
 
 export class EsError extends KbnError {
   constructor(protected readonly err: IEsError) {
@@ -31,13 +31,15 @@ export class EsError extends KbnError {
 
   public getErrorMessage(application: ApplicationStart) {
     const rootCause = getRootCause(this.err)?.reason;
+    const topLevelCause = getTopLevelCause(this.err)?.reason;
+    const cause = rootCause ?? topLevelCause;
 
     return (
       <>
         <EuiSpacer size="s" />
-        {rootCause ? (
+        {cause ? (
           <EuiCodeBlock data-test-subj="errMessage" isCopyable={true} paddingSize="s">
-            {rootCause}
+            {cause}
           </EuiCodeBlock>
         ) : null}
       </>

src/plugins/data/public/search/errors/utils.ts

@@ -26,6 +26,10 @@ export function getFailedShards(err: IEsError) {
   return failedShards ? failedShards[0] : undefined;
 }
 
+export function getTopLevelCause(err: IEsError) {
+  return err.body?.attributes?.error;
+}
+
 export function getRootCause(err: IEsError) {
   return getFailedShards(err)?.reason;
 }