source module (for improve error reporting)

often when a module fails the first thing you want to know is which module is the dominator.

This PR needs to also follow up with error logging that understands sourceModule + moduleName

cc @lukemelia

related: https://github.com/stefanpenner/ember-cli/pull/2155

![screenshot 2014-09-30 22 50 29](https://cloud.githubusercontent.com/assets/1377/4469690/a74742a2-4916-11e4-86dc-41c0d1cecb5e.png)

There's an accidental dedent here :)