if(Sys != null)
{
    Sys.Application.add_init(AppInit); 
}
function AppInit(sender) 
{
    Sys.WebForms.PageRequestManager.getInstance().add_endRequest(requestEndHandler);
}
// This function will handle the end request event
function requestEndHandler(sender, args) 
{
   if( args.get_error() )
   {      
      args.set_errorHandled(true);
      if(varRequestTimeOutMsg)
        alert(varRequestTimeOutMsg); //'The request has timed out. Please refresh the page.'); //show a custom error message.
   }
}