Fetches the error message when manifest upload fails.

Uploading manifest seems to fail frequently when uploading at the
jenkins end. This will help us know the exact error.
Neat. I didn't know you could do this!

I think this can all fit on one line. It's not a blocker, but it'd be nice to have this change made:

```python
        self.assertEqual(
            u'success',
            task['result'],
            u'Following is the error: {0}'.format(task['humanized']['errors'])
        )
```