Retry failed ZFS uploads and downloads automatically

S3 upload timeouts were retried already with an exponential backoff, but
this adds retrying for other kinds of upload failures as well as failed
downloads.  If 5 consecutive failures occur a file sync error is
thrown.

Failed file sync requests to the Zotero API are not currently retried,
but S3 accounts for the majority.

The download portion of this still needs further testing.