Return proper response (not 500) for some face apis

[hooram]

URLs

• /api/facetolabel/
• /api/trainfaces/

should return 200 when there is no labeled faces.

https://github.com/hooram/ownphotos-backend/blob/232f43eec933e591fab48dbe4df37bd181dba04b/api/face_classify.py#L22

https://github.com/hooram/ownphotos-backend/blob/232f43eec933e591fab48dbe4df37bd181dba04b/api/face_classify.py#L52

[luckydonald]

404 not found could make sense too.