The following code snipped demonstrated how you could setup your own custom error handler.

  function customHandler(desciption, page, line, position)  {
    alert(
     'JavaScript error occurred! \n'
     +'\nError description: \t'+description
     +'\nPage address:      \t'+page
     +'\nLine number:       \t'+line
     +'\nPosition:          \t'+position
    )
   
    return true;
  }

  window.onerror = customHandler;

This will display a custom message on screen with information about the JavaScript error that occured.