Provide useful error message when styles/app.[ext] is not found

To avoid future developer confusion as witnessed on issue #1641. This PR provides improved error messaging to assist developers in debugging their applications; it does not fix any actual bugs [since the problem is not really a bug].
Can you throw a [SilentError](https://github.com/stefanpenner/ember-cli/blob/master/lib/errors/silent.js) instead?