Add error handling to #loadJs

Hi,

Thanks a lot for `meteor-external-file-loader`, very useful.

I needed a way to handle error cases (e.g. display an error/retry message to the user if I can't process because they got disconnected or something went wrong with the lib's loading)

Turns out `jQuery.getScript` already returns a promise, so it's pretty straightforward to allow error handling to your lib's user with something like that:

```js
Loader.prototype.loadJs('url', successCallback, 10000).fail(errorCallback)
```

Thought it might be useful to other people too.