bugfix: webpack crashes if error happens in sweetjs compiler

Hi, Pete.  Thanks for useful piece of software.  I'm on my way adopting this into my project (React + hiccup throw sweetjs).  And I've faced with two problems with current implementation:
1. It crashes if there's syntactic error in a processed file. For example:
`
rclass MyView = {
	render: function() {
		hiccup [div#foo.bar.baz {some: "property", another: this.props.anothervalue} 
         [p "A child element"] "Child text"]
	};
}` will crash because of ";" and it will show the exact stack trace as in [Issue #1](https://github.com/petehunt/sweetjs-loader/issues/1). 
2. I want to see compile-time errors in my browser console.
So here's my pull request, if you will.
What if you do `callback(e)`?