Clean up these uploads if error encountered?

[godotgildor]

Currently, we upload the RunInfo.xml and sample sheet in this script. We then call the dx_sync_directory.py. If we encounter an error in that (say it throws an exception) then we will have not uploaded any files except these two. The next time the upload routine occurs, we'll upload these two files again, encounter the error again, an it will carry on ad infinitum. Should we at least consider cleaning up these two files if the call to dx_sync_directory.py fails?

https://github.com/dnanexus-rnd/dx-streaming-upload/blob/8fec7ae047504b6b39996dd52e52dd684306e518/files/incremental_upload.py#L359-L362