POST contributions/ - Return Sensible Error Messages

[Ore4444]

https://github.com/Backfeed/backfeed-restapi/blob/master/restapi/views/contributions.py#L30

The POST contributions/ endpoint should return:

{
    "errorMessage": "user_id parameter is missing"
}

when user_id is missing from the POST object.

It should also return:

{
    "errorMessage": "user_id does not belong to a real user"
}

when get_user(user_id) cannot find a user with that ID.