Prototype: Display 503 error message with Ajax.Request

Question

I'd like to display the error message that comes back from a Prototype JS Ajax.Request call.

new Ajax.Request('ajax.php?action=Set&netboot=' + $('netboot').value + '&root=', {
        on503: NetbootUpdated,
        onSuccess: NetbootUpdated
        });
}

That is my code. I basically want to use on503 to display the content that the file called (ajax.php) sends back, without using a seperate Ajax.Updater call, as the error message might not display the second time the call is sent.