Finally on error

It looks like it already behaves correctly.

I'm a bit unsure about the last two tests:

```javascript

RSVP.reject('original-rejected').finally(function() {
  throw 'finally-callback-rejected';
});

```

Should `on('error')` trigger twice or once? If once which one?  For now I assumed the current behavior, which is it triggers once, and the error triggered is `finally-callback-rejected`.