prompt user to re-run in --verbose to show full exception traceback

since fontmake v2.1.0, we are no longer printing the full exception traceback.
Users may not know that they can run fontmake with `--verbose DEBUG` which will show the full exception traceback.
We need that to be able to debug issues, so I propose we either add that recommendation to a new issue template, and/or print a message like "run with --verbose DEBUG to show the full traceback" when fontmake errors out.