Bootstrapping

What's the advantage in using these `github_status` and `error_handler` functions? Various repos have them, but for [Transition](https://github.com/alphagov/transition/blob/master/jenkins.sh), [Bouncer](https://github.com/alphagov/bouncer/blob/master/jenkins.sh) and the [HMRC manuals API](https://github.com/alphagov/hmrc-manuals-api/blob/master/jenkins.sh) we've been fine with a more minimalist script.