Hook up to subscriptions API

I know I've asked for this before, but can I have an example of an acceptable syntax for v2 related API failures? In v1 we used to have a fail fast method that I've not seen in use since v2... Maybe add it to https://github.com/praekelt/vumi-jssandbox-toolkit/tree/release/0.2.x/examples/http so it's on record?