Don't crash when parsing invalid dates, and mark invalid dates as such.

This is a quick fix to avoid the crash reported on issue #38.

* On `todolib.py`, avoid crashing if the threshold date is invalid;
* On `task_htmlizer.py`, avoid crashing if the due and/or threshold date is invalid, and add a "*** Invalid date format! ***" message so the user know there's something wrong;

I know the ideal solution maybe would involve displaying a proper error dialog, but I couldn't find an easy way to do that from the task list controller.

This was a 10 minute hack, so please feel free to discard it if you decide to implement a more elegant / robust solution.