pgsql: Make a editorial pass over pgbench's error messages.

Make a editorial pass over pgbench's error messages.

The lack of consistency, and lack of attention to our message style
guidelines, was a bit striking.  Try to make 'em better.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/22ba5563adacd162d97ff3c80eac4893574f1e17

Modified Files
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src/bin/pgbench/pgbench.c |  167 ++++++++++++++++++++++++---------------------
1 file changed, 91 insertions(+), 76 deletions(-)