public class Login2 : System.Web.UI.WebControls.Login {

   protected override void OnAuthenticate (System.Web.UI.WebControls.AuthenticateEventArgs e) {
      try {
         base.OnAuthenticate(e); 
      } catch (Exception ex) { 
         HttpContext.Current.Response.Write("ex: " + ex.Message);
         // or HttpContext.Response.Redirect("http://www.google.com");
      }
   }
}

this is the other option...

I hope any of these is what you are looking for, because I think I don't fully understand your needs.