Prevent service from changing the shape of our errors

This ensures a consistent interface for our kibana client errors,
whether the error is raised directly from a `fetch` or emitted from a
search strategy's observable; namely: that both contain this top-level
`body` key.

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

@@ -25,8 +25,11 @@ import { IEsError } from './types';
 import { getRootCause, getTopLevelCause } from './utils';
 
 export class EsError extends KbnError {
+  protected readonly body: IEsError['body'];
+
   constructor(protected readonly err: IEsError) {
     super('EsError');
+    this.body = err.body;
   }
 
   public getErrorMessage(application: ApplicationStart) {