Standardise errors to be more node-like, fixes #5.

I ended up with the following callback logic for all the cases, which I think is more node-like and intuitive.

```js
cpr('from', 'to', function (err, files) {
      // Standard node-like error instance
      if (err) {
         console.log(err.message);
         // err.list is the original errors that were caught when copying
         // Note that this is for advanced usage
         err.list && err.list.forEach(function (err) { console.log('>', err.message); });
     }
});
```

If you guys don't agree, I can always remake to do what I original said.