Parse docker stdout/stderr properly

[norrs]

{u'status': u'Pulling repository docker.example.net:5000/centos'}
{u'errorDetail': {u'message': u'Error: image centos not found'}, u'error': u'Error: image centos not found'}

Should throw an PackagerException if error is found.

[norrs]

I'll test this some more in the weekend hopefully, and probably make another pull request for it.