WebUI: Add handling for HTTP error 404
In case that API is not accessible the 404 error is thrown. There was error dialog
with almost no information. The new dialog says what error is there and what can be
the main cause of the error.
https://fedorahosted.org/freeipa/ticket/4821
Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>