Response error.root_cause shouldn't empty

While playing around with master I received the following error response:

``` json
{
  "error": {
    "root_cause": [],
    "type": "access_control_exception",
    "reason": "access denied (\"java.io.FilePermission\" \"default-mapping.json\" \"read\")"
  },
  "status": 500
}
```

The `root_cause` should probably contain something, even if it is just a copy of the outer level error.
