Make scope_download_classification.py robust to server errors

[bfhealy]

The scope_upload_classification.py and fritz.py code was recently updated to handle certain errors that pertain to brief interruptions in the connection to Fritz. We should do the same for the download script, which currently fails and does not save any output in the case of a single interruption to the connection.

Feature Summary
Added robustness to scope_download_classification.py

Usage / behavior

• Upon a connection error, retry the API call a specified number of times.
• Establish checkpoints to occasionally save downloaded data so all progress is not lost if the download loop is broken.
• Allow the user to continue an interrupted download from a specified source

Implementation details

• The api calls in scope_upload_classification.py and fritz.py have examples of handling server connection errors.