Log offending filename details for build command.

Screenshot of a dummy app (where I left out a comma in an object definition):

![screenshot](http://monosnap.com/image/oJTAOC5MrN7wzO3VFxcmRPTlh1lqwZ.png)

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A few more references as to where `err.file` is coming from:

The filename is being set as `file` on the rejection value by `broccoli-filter` ([here](https://github.com/joliss/broccoli-filter/blob/7346b59b99028b1b514977d687e284b818a54711/index.js#L87)) and `broccoli-es6-concatenator` ([here](https://github.com/joliss/broccoli-es6-concatenator/blob/master/index.js#L119))

https://github.com/joliss/broccoli/pull/102 should cause this to generate nice output for both `broccoli build` and `broccoli serve` commands, but our custom `ember build` task will not take advantage of that PR.