Condition isset($msg->error) added

Trying to fix php error "Undefined property: stdClass::$error" when there is json response but there is no "error" field.
This scenario happens when we're trying to delete a non-existent record (like when SearchableBehavior is added after document is created).

```
> http DELETE 'http://localhost:9200/mywebapplication/minute/1' 
HTTP/1.1 404 Not Found
Content-Length: 83
Content-Type: application/json; charset=UTF-8

{
    "_id": "1", 
    "_index": "mywebapplication", 
    "_type": "minute", 
    "_version": 2, 
    "found": false
}
```
Merge pull request #28 from urmaul/patch-1

Condition isset($msg->error) added