Error when a non-existent (non-void) user is given at login

When a invalid user is given at login, notebook hangs because a LookupError exception is raised. This is due to changes (sage 5.8 -> 5.9) in the exception raised by sagenb.notebook.usermanager.UserManager.user and sagenb.notebook.usermanager.UserManager._user in some cases. This had not be taken into consideration in the code of sagenb.flask_version.authentication.login.

When a void user is given at login the behaviour was correct because this fact raises the old exception. This was taken into account in this request.