WebAccess: IP-based rules and offline user fix

• Correctly handles a corner case of access_control_firerole.acc_firerole_check_user() where, when checking a guest user created with collect_user_info(None) (hence having the 'remote_ip' key set to '') against an IP-based FireRole rule, would have raised an unhandled exception. (closes #1233)

• Introduces a new register_exception() statement in order to early discover troubles with acc_firerole_check_user().