Before this fix, rclone retries chunks of multipart uploads. However
if they had been partially received dropbox would reply with an
incorrect_offset error which rclone was ignoring.
This patch parses the new offset from the error response and uses it
to adjust the data that rclone sends so it is the same as what dropbox
is expecting.
See: https://forum.rclone.org/t/dropbox-rate-limiting-for-upload/29779