pgsql: Fix wrong assertion and poor error messages in "COPY (query) TO"

Fix wrong assertion and poor error messages in "COPY (query) TO".

If the query is rewritten into a NOTIFY command by a DO INSTEAD
rule, we'd get an assertion failure, or in non-assert builds
issue a rather confusing error message. Improve that.

Also fix a longstanding grammar mistake in a nearby error message.

Per bug #18664 from Alexander Lakhin. Back-patch to all supported
branches.

Tender Wang and Tom Lane

Discussion: https://postgr.es/m/18664-ffd0ebc2386598df@postgresql.org

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/6a57a457cf7d3409579868ef17de6725ec6e67ca

Modified Files
--------------
src/backend/commands/copy.c | 8 ++++++--
src/test/regress/expected/copydml.out | 10 +++++++---
src/test/regress/sql/copydml.sql | 4 ++++
3 files changed, 17 insertions(+), 5 deletions(-)