Returning the authenticator on failed login

[bramvdbogaerde]

At the moment the authenticator isn't used in case of a failed login so it makes little sense to let types that implement the Authenticator trait return a reference to themselves when the login has failed.

This makes even less sense if you consider that the authenticator must be able to return a User type and thus must embed that within, but if the authentication failed such type cannot always be constructed.

Thus I propose to let the function return something else for the Error variant of the result.