stats collate_worker catchall

[clayg]

in stats.log_processor.collate_worker, a generic exception in process_one_file will blow up the worker. It would still get logged to stdout if multiprocess and swift.utils.capture_stdio are playing nice together - but p.logger.exception('unable to process %s' % item) would be a lot nicer (stdout exception logging lines get all mixed up). Plus the worker could "continue" on and try the next item in the queue.