webui: fix regressions failed auth messages
1. after logout, krb auth no longer shows "session expired" but correct
"Authentication with Kerberos failed".
2. "The password or username you entered is incorrect." is showed on
failed forms-based auth.
https://fedorahosted.org/freeipa/ticket/5163
Reviewed-By: Martin Basti <mbasti@redhat.com>