Handle minify errors metalsmith way.

How errors should be handled is described here: http://www.robinthrift.com/posts/metalsmith-part-3-refining-our-tools/#a-note-error-handling

Doing that, metalsmith's `build(function(err, files) {})` can handle it. Currently if minifying fails for a reason, it is impossible to catch it through metalsmith pipeline. Whole application stops exit an exit code other than 0.