Fix ALTER TABLE error message

Fix ALTER TABLE error message

This bogus error message was introduced in 2013 by commit f177cbfe676d,
because of misunderstanding the processCASbits() API; at the time, no
test cases were added that would be affected by this change.  Only in
ca87c415e2fc was one added (along with a couple of typos), with an XXX
note that the error message was bogus.  Fix the whole, add some test
cases.

Backpatch all the way back.

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://postgr.es/m/202503041822.aobpqke3igvb@alvherre.pgsql