Report errors using Sentry

One of the biggest problems when developing downloadable software is collecting errors. We can hope that people will create bug reports, but many times they don't have Github accounts and don't know where to go. 

To fix this, we're going to use [Sentry](http://www.getsentry.com). I've already talked to the folks who run the service, and they've given us a free OS account, which we can make public.