window.onerror does not work

I have some tricky AJAX code on a form, and sometimes it will fail (don't ask why, I can't get around it). When this happens, I need to 开发者_开发技巧trap the error, reset a hidden field indicator, and submit the form naturally so that the user does not have an unpleasant experience. I planned on using window.onerror to do this, but it is never firing! I am using IE8 and all I have to worry about is the IE browser. Is there some gotcha to getting this event to work? Here's my code...

window.onerror = function() {
  alert("Error!");
  document.getElementById("hidAjax").value = "0";
  document.forms[0].submit();
}

"A common problem that bites many developers occurs when their onerror handler is not called because they have script debugging enabled for Internet Explorer. This will be the case by default if you have installed the Microsoft Script Debugger or Microsoft Visual Studio 6.0® (specifically Visual InterDev 6.0™)—onerror handling is how these products launch their debugger. You can disable script debugging for a given instance of Internet Explorer on the Advanced tab of the Internet Options dialog box (note that checking the Disable script debugging setting will apply only to that instance of Internet Explorer):"

http://msdn.microsoft.com/en-us/library/ms976144.aspx

try/catch also introduces an additional error object that only has the scope of the catch. In applications where performance matters, this is not a good idea.

Any reason not to just put a try/catch around the tricky code?