pgbench: Fix assertion failure with retriable errors in pipeline mode.

pgbench: Fix assertion failure with retriable errors in pipeline mode.

When running pgbench with --verbose-errors option and a custom script that
triggered retriable errors (e.g., serialization errors) in pipeline mode,
an assertion failure could occur:

    Assertion failed: (sql_script[st->use_file].commands[st->command]->type == 1), function commandError, file pgbench.c, line 3062.

The failure happened because pgbench assumed these errors would only occur
during SQL commands, but in pipeline mode they can also happen during
\endpipeline meta command.

This commit fixes the assertion failure by adjusting the assertion check to
allow such errors during either SQL commands or \endpipeline.

Backpatch to v15, where the assertion check was introduced.

Author: Yugo Nagata <nagata@sraoss.co.jp>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwGWQMOzNkQs-LmpDHdNC0h8dmAuUMRvZrEntQi5a-b=Kg@mail.gmail.com