Catch invalid datetime string as Validation error (400) instead of inter...

...nal server error (500)

TL;DR
Return a 400 when invalid datetime string is given.

Currently when I post a datetime string, for example "0000-00-00", I receive a 500 returncode, which is equal to an internal server error. The traceback of flask shows "ValueError: month must be in 1..12".

What makes more sense to me is a 400 returncode with a Validation Error message.