REST query failures should give meaningful error messages

[ghost]

When the REST query for the Streams instance fails, kKeyError: 'requestObj' together with a Python stack trace is displayed. The REST response in such a case can be like this:

{
  "_messageCode_": "Not Found",
  "_statusCode_": 404,
  "exception": "ServiceInstanceRecordNotFound",
  "message": "Not Found"
}

The rest primitive code should evaluate the message code and continue only in case of _messageCode_ = success instead of running into KeyError when the JSON response is accessed.