Always throw getUser error

Whenever a user which is not logined visit the site, it throws a getUser error. Why is `userauth` [trying to login directly before redirect to login path](https://github.com/koajs/userauth/blob/master/index.js#L179-L185)? In my case, it will always fail.

Start by reading index.js lines 179-185 and reproduce the getUser error for an unauthenticated visitor. Trace the attempted login and redirect-to-login path behavior; done means unauthenticated visits reach the login path without throwing the getUser error.

Written by the indexing model from the issue text.