Some small polishing of Mark Hollomon's cleanup of DROP command: might

as well allow DROP multiple INDEX, RULE, TYPE as well.  Add missing
CommandCounterIncrement to DROP loop, which could cause trouble otherwise
with multiple DROP of items affecting same catalog entries.  Try to
bring a little consistency to various error messages using 'does not exist',
'nonexistent', etc --- I standardized on 'does not exist' since that's
what the vast majority of the existing uses seem to be.