title: Detect failure to load contents of an iframe
body: <p>I can detect when the content of an iframe has loaded using the <em>load</em> event. Unfortunately, for my purposes, there are two problems with this:</p>
  
  <ul>
  <li>If there is an error loading the page (404/500, etc), the load event is never fired.</li>
  <li>If some images or other dependencies failed to load, the load event is fired as usual.</li>
  </ul>
  
  <p>Is there some way I can reliably determine if either of the above errors occurred?</p>
  
  <p>I'm writing a semi-web semi-desktop application based on Mozilla/XULRunner, so solutions that only work in Mozilla are welcome.</p>