Return 404 if no face was detected

[luckydonald]

Hey, awesome working docker container!

As it should be possible to know if a face was detected.
One way would be to return 404 status code if no faces were found.
As the current behavior is to return it (maybe resized), it would probably need a flag like ?require_detection=true?

Also possible would be

• No faces detected: return image, 200 OK
• was able to add a mustache: return image, 201 Created