While fixing plperl and pltcl, I realized plpgsql wasn't doing

While fixing plperl and pltcl, I realized plpgsql wasn't doing
subtransactions quite right either: the ReleaseCurrentSubTransaction
call should occur inside the PG_TRY, so that the proper path is taken
if an error occurs during subtransaction commit.  This assumes that
AbortSubTransaction can cope with the state left behind if
CommitSubTransaction fails partway through, but we were already
requiring that.