ElasticSearch: Improve ES error handling message (#61471)

Look for 'caused_by.reason' in ES error response

When the ES response does not contain `reason` or `root_cause[0].reason`
is empty, there is no information for the user to know what is going
wrong.

An example of the error message after this change:
```
Failed to evaluate queries and expressions: failed to execute query A: Trying to create too many buckets. Must be less than or equal to: [65536] but this number of buckets was exceeded. This limit can be set by changing the [search.max_buckets] cluster level setting.
```

Related to https://github.com/grafana/grafana/issues/61246

pkg/tsdb/elasticsearch/response_parser.go

@@ -711,12 +711,15 @@ func getErrorFromElasticResponse(response *es.SearchResponse) string {
 	json := simplejson.NewFromAny(response.Error)
 	reason := json.Get("reason").MustString()
 	rootCauseReason := json.Get("root_cause").GetIndex(0).Get("reason").MustString()
+	causedByReason := json.Get("caused_by").Get("reason").MustString()
 
 	switch {
 	case rootCauseReason != "":
 		errorString = rootCauseReason
 	case reason != "":
 		errorString = reason
+	case causedByReason != "":
+		errorString = causedByReason
 	default:
 		errorString = "Unknown elasticsearch error response"
 	}