Only return invalid user id when it's the only error

[danschultzer]

Idea for resolving #170. Not sure about this one as it seems there will be plenty of edge cases to this. The better way of handling this is probably to add a way to more easily manage auth flow. This ties in with my comment in https://github.com/danschultzer/pow/issues/234#issuecomment-513581445 where you have a plug like interface to handle auth flow in one module.