Prevent authentication redirect loop during login failure

Imagine this scenario:

1. Anonymous user visits a page that is restricted in some way, which returns a 403 access denied.
2. Your site has the r4032login module enabled (or some custom solution) which automatically redirects anonymous users to login via CAS when accessing a restricted page.
3. User is redirected to the CAS server for login
4. CAS server redirects user back to your site for verification after the login process.
5. There's a failure in the verification process. Either verification of the CAS service ticket fails, or some module prevented user login using one of the login/registration events this module dispatches.
6. The CAS module adds an error status message indicating that login failed, and then redirects the user to the page they were originally on when attempting login
7. Process begins again at step 1, creating an infinite loop.

To fix this, we need to send users to a page that doesn't require authentication after a login failure. The page that makes the most sense is the /user/login page. But we should make it configurable, in case someone has a custom alternate login page.