Set up error logging

[joepio]

I'd like to suggest that errors in production are logged somewhere. We have a bunch of nasty bugs that seem hard to track without logs.

I've heard good things about Highlight.io (docs for adding to python), basically an open source alternative to bugsnag / sentry. I've already set up an account there, but I won't add it to this repo (too much stuff on my plate, sorry).