Use this.emit('error', ...) so that the error can be handled by the gulp stream

This allows us to add an `.on('error', handler)` after calling the sweet plugin and then output a nice error instead of a stacktrace.

![screenshot](http://content.screencast.com/users/max.peterson/folders/Jing/media/eabbe381-68fb-4efe-8797-366f1d2cb78c/00001157.png)

@toranb