Handle closing a PR

@sarina is this what you were hoping for?
I've currently got monitoring set up via [Bugsnag](https://bugsnag.com), which has a free tier. Unfortunately, one of the limitations of their free tier is that you can only have one user receiving notifications when things go wrong (the owner of the Heroku app). So, Bugsnag sends me an email whenever an exception is raised. I could set up an email rule to auto-forward that email to whoever is interested, though.