high values for client_min_messages

The patch updates the documentation to reflect the fact that higher values
of client_min_messages (fatal + panic) are valid and also fixes a slight
issue with how psql tried to display error messages that aren't sent to
the client.

We often tell people to ignore errors in response to requests for things
like "drop if exists", but there's no good way to completely hide this
without upping client_min_messages past ERROR.  When running a file like

SET client_min_messages TO 'FATAL';

DROP TABLE doesntexist;

with "psql -f filename" you get an error prefix of
"psql:/home/username/filename:3" even though there is no error message to
prefix because it isn't sent to the client.

Kris Jurka