default URI getUnauthorizedRedirectUrl() {
        return null;
    }
Redirect the request to a separate and possibly external URL
in case authorization fails for this service. If no URL is
specified, CAS shall redirect the request by default to a generic
page that describes the authorization failed attempt.

@return the redirect url
@since 4.2
api/cas-server-core-api-services/src/main/java/org/apereo/cas/services/RegisteredServiceAccessStrategy.java