Make robust for "too many connections" errors

Important since web_trawler actively uses multiprocessing and -threading.

This might be useful reads here

https://stackoverflow.com/questions/3490173/how-can-i-speed-up-fetching-pages-with-urllib2-in-python

https://stackoverflow.com/questions/16181121/a-very-simple-multithreading-parallel-url-fetching-without-queue

https://pawelmhm.github.io/asyncio/python/aiohttp/2016/04/22/asyncio-aiohttp.html

added bug label

Web_trawler already uses both multiprocessing and multithreading. I think all that's needed is to catch certain kinds of errors, time.sleep for a while, and try again (perhaps with a ceiling to how many retries).