Debug false success in parallel check/extract

[forsyth2]

Initially reported in #167 with partial fix in #168.

zstash check was reporting success even if exceptions occurred. This is related to the multiprocessing. Code blocks to investigate would include the following (all in https://github.com/E3SM-Project/zstash/blob/master/zstash/extract.py):

In multiprocess_extract:

    failures: List[FilesRow] = []
    while any(p.is_alive() for p in processes):
        while not failure_queue.empty():
            failures.append(failure_queue.get())

In extractFiles:

        except Exception:
            # Catch all exceptions here.
            traceback.print_exc()
            logger.error("Retrieving {}".format(files_row.name))
            failures.append(files_row)

    if multiprocess_worker:
        # If there are things left to print, print them.
        multiprocess_worker.print_all_contents()

        # Add the failures to the queue.
        # When running with multiprocessing, the function multiprocess_extract()
        # that calls this extractFiles() function will return the failures as a list.
        for f in failures:
            multiprocess_worker.failure_queue.put(f)