ERROR messages in VACUUM's PARALLEL option

Over in [1], Horiguchisan mentioned a few things about VACUUM's new
BUFFER_USAGE_LIMIT option.

1) buffer_usage_limit in the ERROR messages should be consistently in uppercase.
2) defGetString() already checks for opt->args == NULL and raises an
ERROR when it is.

I suspect that Melanie might have followed the lead of the PARALLEL
option when she was working on adding the BUFFER_USAGE_LIMIT patch.

What I'm wondering now is:

a) Is it worth changing this for the PARALLEL option too? and;
b) I see we lose the parse position indicator, and;
c) If we did want to change this, is it too late for v16?

For a), I know that's much older code, so perhaps it's not worth
messing around with the ERROR messages for this. For b), this seems
like a fairly minor detail given that VACUUM commands are fairly
simple. It shouldn't be too hard for a user to see what we're talking
about.

I've attached a patch to adjust this.

David

[1] https://postgr.es/m/20230411.102335.1643720544536884844.horikyota.ntt@gmail.com