pgsql: 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

Branch
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REL_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4e026be5f15994a7036eaca172126226e053f655

Modified Files
--------------
src/backend/parser/gram.y                 | 2 +-
src/test/regress/expected/foreign_key.out | 6 +++++-
src/test/regress/sql/foreign_key.sql      | 4 +++-
3 files changed, 9 insertions(+), 3 deletions(-)