Refactor error messages to reduce duplication

Alvaro Herrera <alvherre@alvh.no-ip.org>

Commit: 498ee9ee2f4bc7d79f2d91cdd817b2a8f14a664f
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2024-08-08T19:17:11Z
Releases: 18.0
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.

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