Catch missing bake and debug kit plugins

Otherwise the app won't actually work in production, without either the
development-plugins being added or user modifying their bootstrap files.
(users _should_ modify their bootstrap files - but it's likely a
significant number won't and be confused by the missing plugin errors if
they remove the development-only plugins).
As mentioned elsewhere - I think so, but rather than create merge conflicts am waiting for [that conversation](https://github.com/cakephp/app/pull/192#issuecomment-67001589) to reach a conclusion.