Add the abort_upload, delete_file and move_file handlers.

- Add the 'remove_upload_id' helper function

- Changed the get_chunk 'files/dropbox' appending to be done to every
  filename. It wasn't done if the stored filename was already beginning
  by this string, but this could've been error-prone if the user decided
  to reproduce the same file tree & folder names

- Manage the 'has_more' corner case with deltas when checking changes

- Add more error handling with the ServiceError exception