Return an elasticsearch error response for elastic search endpoint

[fmassot]

Expected JSON is:

{
   "status": 400
   "error": {
      "type": "xxx",
      "reason": "",
      "stackTrace": "",
      "causedBy": {
         "reason": ...
      },
      "rootCause": [{...}]
   }
}

For example, the Elasticsearch grafana plugin is trying to get the message by getting the information in the following order:

• root_cause[0].reason
• reason
• caused_by.reason

We could start by just putting the message error in the reason field.

Java client Ref