running with non-empty directory doesn't give clear error
peewee.IntegrityError: UNIQUE constraint failed: rubric.key
Actually, this is possibly a newer Plom problem: the db creation code does a bunch of rubric stuff now: perhaps your are legit trying to hot-start in non-empty dir...? And Andrew and I have broken things... Not sure.
But if its not supposed to work, then maybe assert or at least warn on directory nonempty.