Instead of printing error about REMOTE_USER, redirect to login

When accessing a page that requires authentication and no validated user is authenticated by the environment, the app/webroot/auth/login/index.php page prints an error and exits.

Instead, it should just redirect to the root.

This happens if your browser has remembered an old page and you reopen the browser. Your session was lost, your remote user is unauthenticated (ie: not set), so you should just log on again. Now you have to manually adjust the URL to get to the root and the login button.