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.
It does. But instead of telling me "Unexpected token ;" or something else it prints this
```
/home/bulat/work/hc/wp/node_modules/async/lib/async.js:30
            if (called) throw new Error("Callback was already called.");
                              ^
Error: Callback was already called.
  at /home/bulat/work/hc/wp/node_modules/async/lib/async.js:30:31
  at /home/bulat/work/hc/wp/node_modules/async/lib/async.js:251:21
  at /home/bulat/work/hc/wp/node_modules/sweetjs-loader/index.js:20:21
  at /home/bulat/work/hc/wp/node_modules/webpack/node_modules/enhanced-resolve/lib/UnsafeCachePlugin.js:29:4
  at onResolved (/home/bulat/work/hc/wp/node_modules/webpack/node_modules/enhanced-resolve/lib/Resolver.js:39:10)
  at /home/bulat/work/hc/wp/node_modules/webpack/node_modules/enhanced-resolve/lib/Resolver.js:104:21
  at /home/bulat/work/hc/wp/node_modules/webpack/node_modules/enhanced-resolve/lib/Resolver.js:209:15
  at applyPluginsParallelBailResult.createInnerCallback.log (/home/bulat/work/hc/wp/node_modules/webpack/node_modules/enhanced-resolve/lib/Resolver.js:91:30)
  at loggingCallbackWrapper (/home/bulat/work/hc/wp/node_modules/webpack/node_modules/enhanced-resolve/lib/createInnerCallback.js:20:19)
  at /home/bulat/work/hc/wp/node_modules/webpack/node_modules/tapable/lib/Tapable.js:138:6
  at innerCallback (/home/bulat/work/hc/wp/node_modules/webpack/node_modules/enhanced-resolve/lib/Resolver.js:79:37)
  at loggingCallbackWrapper (/home/bulat/work/hc/wp/node_modules/webpack/node_modules/enhanced-resolve/lib/createInnerCallback.js:20:19)
  at /home/bulat/work/hc/wp/node_modules/webpack/node_modules/tapable/lib/Tapable.js:138:6
  at Tapable.<anonymous> (/home/bulat/work/hc/wp/node_modules/webpack/node_modules/enhanced-resolve/lib/DirectoryDescriptionFileFieldAliasPlugin.js:96:26)
  at /home/bulat/work/hc/wp/node_modules/webpack/node_modules/enhanced-resolve/lib/DirectoryDescriptionFileFieldAliasPlugin.js:48:26
  at Storage.finished (/home/bulat/work/hc/wp/node_modules/webpack/node_modules/enhanced-resolve/lib/CachedInputFileSystem.js:38:16)
  at fs.js:266:14
  at /home/bulat/work/hc/wp/node_modules/webpack/node_modules/enhanced-resolve/node_modules/graceful-fs/graceful-fs.js:104:5
  at Object.oncomplete (fs.js:107:15)
```

If I use console.error() it will print this:
```
{ [Error: Line 6: Unexpected token ;
[... ) } ; } ) ; ...]]
  index: 168,
  lineNumber: 6,
  column: 3,
  description: 'Unexpected token ;' }
```