If possible I would prefer a pop-up instead of an alert. Alerts are not working very well with long tracebacks and the content is easy to copy (i.e. using the traceback for debug).

I would use something like
```javascript
var w = window.open('', 'Traceback', 'width=400,height=400,resizeable,scrollbars');
w.document.write('<pre>'+ traceback+'</pre>');
w.document.close();
```