When files failed to download, usually from contenttoshorts, the traceback:

Traceback (most recent call last):
  File "/usr/local/bin/flickrbackup.py", line 506, in <module>
    main()
  File "/usr/local/bin/flickrbackup.py", line 466, in main
    success = backup.download(ids, arguments.error_file)
  File "/usr/local/bin/flickrbackup.py", line 303, in download
    return self.retry(items_with_errors, error_file=error_file)
TypeError: 'int' object is not callable

occurs. Borrowing from the mycon/flickrbackup fork, make a clearer indication
between self.retry and self.retry()