improve error messages when a module fails to import

continuation of: https://github.com/joliss/broccoli-es6-concatenator/pull/25

cc @lukemelia 


![screenshot 2014-09-30 22 50 29](https://cloud.githubusercontent.com/assets/1377/4469693/c589ad18-4916-11e4-9feb-d10204d4fa8e.png)