title: How to react to failed thumb image loading in TTThumbsViewController?
body: <p>I'm currently using TTThumbsViewController in my project. I'm getting all the urls for TTPhotoVersionLarge and TTPhotoVersionThumbnail from the web so I can't tell that a url for the thumb images will work or not.</p>
  
  <p>Currently the TTThumbsViewController will just display an empty image if the thumb url can't be loaded.</p>
  
  <p>So I want to be notified if a thumb fails to load and do extra error handling when that happens like:</p>
  
  <ol>
  <li>Try to load the url for TTPhotoVersionLarge</li>
  <li>If that fails again display an error image (which is included in the bundle)</li>
  </ol>
  
  <p>I have looked into the three20 code but can't find a proper place where I can implement this proper error handling.</p>