Throw when fs.stat callback provides error

In relation to #300 

There seems to be some error getting file stats on the minified file, probably due to some problem further up the pipeline, resulting in a missing file or something.

This PR just increases the visibility of the error, though you may want to dig into `err` and see if it contains any meaningful information rather than just throwing it away and exploding like this PR does.

@XhmikosR Let me know if this sheds any light on the issues you're seeing on Windows.