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
Would you keep using the `errors` library that we've built? In this case, `errors.NotFoundError` like the original code. I'm not sure what error type would be most appropriate for the user status, but hopefully that lib's well doc'd :D