* Make callback for upload file completion
Callback does not indicate that a file will be reachable immediately in
the resulting snapshot, but does indicate that the uploader is done
processing the file in some way (either via uploading data or finding a
previous version in the repo) and whether there was an error processing
the file.
* Tests for new FinishedFile callback
Ensure hadErr is properly populated and FinishedFile is called even if
the file was considered cached.
* Refine comment on interface function slightly
* Give callback error instead of bool about error
* Add locks around concurrent accesses in test
5c88bcf1a6