pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/68a2a437f46716efd54d177edf9dbcc9916c903b

Modified Files
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src/backend/commands/vacuum.c        | 33 ++++++++-------------------------
src/test/regress/expected/vacuum.out |  6 +++---
2 files changed, 11 insertions(+), 28 deletions(-)