Two concurrent Wants for the same release-group both pass the
findUnique existence check as absent; the loser's create() then hits
the rgMbid unique constraint and threw an uncaught P2002, 500ing the
request. Wrap the create in try/catch and fall back to the same
update-and-return-200 path on P2002, converging instead of erroring.
Non-P2002 errors are rethrown unchanged.
Jonathan
44c1b1a62a