Refactor error messages to reduce duplication

I also took the liberty of changing

	errmsg("COPY DEFAULT only available using COPY FROM")
to
	errmsg("COPY %s cannot be used with %s", "DEFAULT", "COPY TO")

because the original wording is unlike all other messages that indicate
option incompatibility.  This message was added by commit 9f8377f7a279
(16-era), in whose development thread there was no discussion on this
point.

Backpatch to 17.

0 files changed, 0 insertions, 0 deletions