Change onError(Exception) to onError(Throwable) - Issue #296

This changes `Observer#onError(Exception e)` to `Observer#onError(Throwable e)`

See "Observer#onError should use Throwable" https://github.com/Netflix/RxJava/issues/296 for discussion that led to this change.

This is a breaking change and will result in a version bump from 0.9.x to 0.10.x.