The onerror mechanism is kind of new, so it isn't supported in older browsers like safari 3. The workarounds suggested all require checking something expected in the script, and this is meant to be more general purpose.
on Windows, onFail never returns. in Safari 3, the onload callback is fired. http://gwt-code-reviews.appspot.com/1451818/ -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
