pgbench: Improve error-handling in pgbench.

pgbench: Improve error-handling in pgbench.

Previously failures of initial connection and logfile open caused pgbench
to proceed the benchmarking, report the incomplete results and exit with
status 2. It didn't make sense to proceed the benchmarking even when
pgbench could not start as prescribed.

This commit improves pgbench so that early errors that occur when
starting benchmark such as those failures should make pgbench exit
immediately with status 1.

Author: Yugo Nagata
Reviewed-by: Fabien COELHO, Kyotaro Horiguchi, Fujii Masao
Discussion: https://postgr.es/m/TYCPR01MB5870057375ACA8A73099C649F5349@TYCPR01MB5870.jpnprd01.prod.outlook.com