Improve ereports for VACUUM's BUFFER_USAGE_LIMIT option

Improve ereports for VACUUM's BUFFER_USAGE_LIMIT option

There's no need to check if opt->arg is NULL since defGetString() already
does that and raises an ERROR if it is.  Let's just remove that check.

Also, combine the two remaining ERRORs into a single check.  It seems
better to give an indication about what sort of values we're looking for
rather than just to state that the value given isn't valid.  Make
BUFFER_USAGE_LIMIT uppercase in this ERROR message too.  It's already
upper case in one other error message, so make that consistent.

Reported-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/20230411.102335.1643720544536884844.horikyota.ntt@gmail.com