Get rid of bare except:s
Add six version requirement

six.add_metaclass isn't available in all versions of six.
Allow AttributeError when updating NET_WM_DESKTOPS
fix pango markup parsing/rendering
Don't crash on widget errors

We could return "" here, but it seems more useful to let users know that a
particular widget is having problems. We can change this to "" if it becomes
more of an issue. Whatever we do, we can't return None, because we immediately
try to parse None for pango markup and fail.