add an 'auth failure' page - server_auth.php should 401 to this

The intent is that the server be configured to use this page
as its ErrorDocument 401 for server_auth.php. When server auth
fails, this page will be displayed, simply a manual redirect
to tell the user to go try again. It should pick up the correct
URL to link to from the request_url parameter but it doesn't
seem to, I'll have to check into that.

The 'oc_http_auth_status' tracker will ensure that OC fails
isSessionActive() and so hits the password auth page when the
user returns. In theory, I believe, this page could run a small
Javascript which waited a couple of seconds then did a client-
side redirect to avoid the manual step.