pgbench: Don't fail during startup

In pgbench, report, but ignore, any errors returned when attempting to
vacuum/truncate the default tables during startup.  If the tables are
needed, we'll error out soon enough anyway.

Per discussion with Tatsuo, David Rowley, Jim Nasby, Robert, Andres,
Fujii, Fabrízio de Royes Mello, Tomas Vondra, Michael Paquier, Peter,
based on a suggestion from Jeff Janes, patch from Robert, additional
message wording from Tom.