error return fileName

Hi,

A little addition to send an error with a `fileName` property. Exactly like `gulp-uglify` here :
 https://github.com/terinjokes/gulp-uglify/blob/master/index.js#L55

It's usefull if you want notice where the error is, example:
```js
var r = rework(rework.mixin(mixins), vars(), shade());
r.on('error', function(err) {
  console.log('Rework error with file', err.fileName);
  r.end();
})
```