pgbench: make verbose error messages thread-safe

Hi,

While running pgbench with multiple threads and --verbose-errors,
I found that some verbose error messages were corrupted.
This issue happens because printVerboseErrorMessages() uses
a function-local static PQExpBuffer for building those messages.
Since that buffer is shared across threads, it is not thread-safe.

Attached patch fixes this issue by changing printVerboseErrorMessages()
to use a local PQExpBufferData instead of a static one. Thoughts?

Since this issue was introduced in v15, the patch should be
backpatched to v15 if accepted.

Regards,

--
Fujii Masao