Inconsistent style in pgbench's error messages

I made a pass over pgbench's error messages to try to make them meet
project style guidelines. There was one rather large bit of inconsistency
that I didn't try to fix, though: something like half of the messages
prepend "pgbench: " to the front, but the other half don't. Worse yet,
some of the other half prepend other things like metacommand names, which
seems quite confusing if you're expecting the former convention.

I'd like to see that become consistent, but I am not sure which way we
want to jump. Personally I think "pgbench: " adds very little and so
I'd rather get rid of it, but I'm sure others feel differently.

Another issue here is that when dealing with custom scripts, IMO it would
be really helpful to provide the file name, line number, and perhaps line
text whenever a message is about an identifiable command. That's mostly
not happening at present, except for the aforesaid inconsistent mentions
of metacommand names. To make that happen, we'd need to agree on a
formatting convention for attaching the info. I'd be inclined to put it
on a separate line, but beyond that have no specific thoughts.

Comments?

regards, tom lane