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 9f8377f7a2
(16-era), in whose development thread there was no discussion on this
point.
Backpatch to 17.
28b953f263