pg_waldump: Improve option parsing error messages

pg_waldump: Improve option parsing error messages

I rephrased the error messages to be more in the style of
option_parse_int(), and also made use of the new "detail" message
facility.  I didn't actually use option_parse_int() (which could be
used for -n) because libpgfeutils wasn't used here yet and I wanted to
keep this just to string changes.  But it could be done in the future.