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
The trouble is that the NotFoundError doesn't contain a human-readable message. We could work on NotFoundError itself, but you currently have to catch the error in-flight and return a human-readable message to the notification util (**which is already happening in the user model**: https://github.com/TryGhost/Ghost/blob/master/core/server/models/user.js#L507)