pgbench: Report errors during run better

pgbench: Report errors during run better

When an error occurs during a benchmark run, exit with a nonzero exit
code and write a message at the end.  Previously, it would just print
the error message when it happened but then proceed to print the run
summary normally and exit with status 0.  To still allow
distinguishing setup from run-time errors, we use exit status 2 for
the new state, whereas existing errors during pgbench initialization
use exit status 1.

Reviewed-by: Fabien COELHO <coelho@cri.ensmp.fr>