Allow tmdb3.py to use $MYTHCONFDIR as a cache location.

This makes tmdb3.py follow the standard behavior of using MYTHCONFDIR,
falling back to HOME, for its selection of scratch space. This also adds
explicit error reporting to stdout when something goes wrong.

Refs #11876

(cherry picked from commit 2c50a824be1f3083ca966ff0309ea39854452b29)