Add button to display a popup with the error traceback, if a calculation fails

Perhaps this approach is a bit too naive. It simply checks the status of the calculation and, if the calculation fails, it adds a button to the available "actions". If the user clicks it, a popup window containing the traceback of the error is displayed.

Things to be improved:
* the traceback is ugly (good for debugging, but absolutely not user-friendly). I suppose we should investigate if there's any nicer way to provide information to the user, maybe at least pre-formatted in some way;
* I've simply given class="btn" to the button, which displays it as the other buttons, with a grey color. The color might be changed, for instance, into yellow (still unused by the other buttons), but yellow might look like a "warning", so I don't know if it's appropriate. Maybe a darker grey, then? Any suggestions?