Re: Disallow setting client_min_messages > ERROR?

OK, so the consensus seems to be that the back branches should continue
to allow you to set client_min_messages = FATAL/PANIC, but then ignore
that and act as though it were ERROR.

We could implement the clamp either in elog.c or in a GUC assignment
hook.  If we do the latter, then SHOW and pg_settings would report the
effective value rather than what you set.  That seems a bit cleaner
to me, and not without precedent.  As far as the backwards compatibility
angle goes, you can invent scenarios in which either choice could be
argued to break something; but I think the most likely avenue for
trouble is if the visible setting doesn't match the actual behavior.
So I'm leaning to the assign-hook approach; comments?