Update download.py

[saunders-jake]

For the USCG openctf, there were some old challenges that were causing issues. I didn't look too closely but it seems the API was returning a json object without a "description" key sometimes, which caused the script to have a KeyError. This patch uses .get(blah, "N/A"), so that they key is created with the value "N/A" if the description key isn't sent.