Signin: Proper notification if user not found

closes #3374
- If user object is returned but undefined, we'll display a
human-readable
error notification (user model)
- If user object is returned, but the user is inactive or invited (but
not
activated), we'll display a human-readable error notification
Merge pull request #3386 from felixrieseberg/iss3374

Signin: Proper notification if user not found