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