Handle rate limit errors

Hopefully, this PR will catch the quota errors and die gracefully, caching the work that was done so far. Then the next time the script runs, it wont have to start from scratch again. After running and dying a couple of times, this will hopefully be able to handle larger inboxes.

@gregose @mhagger would you mind giving this a shot?
https://github.com/mastahyeti/OctoGAS/issues/4