Add a default msg to show the response content. (#1064)

* Add a default msg to show the response content.

This seems like an issue with using assertResponseNoErrors and assertResponseHasErrors 

Which doesn't include any errors specific to the response and currently just shows.

```python
    self.assertNotIn("errors", list(content.keys()))
AssertionError: 'errors' unexpectedly found in ['errors', 'data']
```

* Update testing.py

Author: jackton1

graphene_django/utils/testing.py

@@ -109,14 +109,14 @@ def assertResponseNoErrors(self, resp, msg=None):
         the call was fine.
         :resp HttpResponse: Response
         """
-        self.assertEqual(resp.status_code, 200)
         content = json.loads(resp.content)
-        self.assertNotIn("errors", list(content.keys()), msg)
+        self.assertEqual(resp.status_code, 200, msg or content)
+        self.assertNotIn("errors", list(content.keys()), msg or content)
 
     def assertResponseHasErrors(self, resp, msg=None):
         """
         Assert that the call was failing. Take care: Even with errors, GraphQL returns status 200!
         :resp HttpResponse: Response
         """
         content = json.loads(resp.content)
-        self.assertIn("errors", list(content.keys()), msg)
+        self.assertIn("errors", list(content.keys()), msg or content)