[DO NOT MERGE] Test_cv_subscribe_system

Third time the charm
Because I do:
```
if 'error' in response or 'errors' in response:
            message = response.get('error') or response.get('errors')
            raise FactoryError(
                'Error encountered while POSTing to {0}. '
                'Error received: {1} '
                'Status Code: {2} '
                ''.format(raw_response.url, message, raw_response.status_code)
            )
```
and you test it, and for the test to pass, MockErrorResponse needs to contain status and url.