onCompleted swallows all exceptions

I was fairly surprised by the behavior of this stream:

``` java
Observable.empty()
    .subscribe(
        System.out::println, 
        System.err::println,
        () -> { throw new RuntimeException(); }
    );
```

What I expected to see was the thread crashing due to the exception; instead absolutely nothing happens because `onCompleted` swallows the exception entirely.
