Don't crash when removing a user that's not project member

The attempt to revoke project access for a user that was not member of the
project results in a 500 Internal Server error where it actually should
result in a 200 OK since after the operation, the user is not member of
the project. This turns the operation into an idempotent call that can
be repeated with no ill effects.

Updated the spec and changed the code accordingly. However, the result differs
slightly, as we can't return the users project access level if the user was not
member. I'm not aware if anybody relies on the result of this call.

Fixes #2832