Stop trying to raise custom error for missing DSL method

It was ruining the backtrace and making bugs harder to find.
Prime app from manifest file

In non-development modes, an app that has been previously processed
leaves a .assets.{env}.json file in the output directory. This file is
then used to prime the app on restart in the relevant env.
Setup cukes to run features, then wip

I keep breaking the JSON API feature because it was added in haste.
Tagged as wip until the other features are stable and I've decided how
it should behave.