Fix error returned from login plugins

Some login plugins use form based authentication and let the user retry
on authentication errors. This is fine, however the wrong error code is
returned in this case, 401 should be returned.

Fixes: https://fedorahosted.org/ipsilon/ticket/94

Signed-off-by: Simo Sorce <simo@redhat.com>
Reviewed-by: Nathan Kinder <nkinder@redhat.com>