Detect failure to load contents of an iframe

I can detect when the content of an iframe has loaded using the load event. Unfortunately, for my purposes, there are two problems with this:

• If there is an error loading the page (404/500, etc), the load event is never fired.
• If some images or other dependencies failed to load, the load event is fired as usual.

Is there some way I can reliably determine if either of the above errors occurred?

I'm writing a semi-web semi-desktop application based on Mozilla/XULRunner, so solutions that only work in Mozilla are welcome.