Is it possible to display a custom message when a user fails authentication via ACS? We deny users access to the web if they do not have a proxy server configured. If they remove the proxy settings the firewall will prompt them and display a username and password from ACS (this allows service staff to bypass). Normal users keep trying (and failing) but the default 'Authentication Failed' message is not enough to inform them of what they should be doing.
I have this problem too.