Return more information on 'no face detected' error

[kboyington]

When training a face for facial recognition, sometimes Deepstack rejects the request to '/v1/vision/face/recognize' with {'success': False, 'error': 'no face detected'} but does not give us information on which file failed.

It would be more helpful if the API returned which photo it did not detect a face in. That was the programer can remove that photo from the set.