Invoking cancellation callback in onCancelled()

I think we should do this method final, to ensure the common flow is followed by all the extending classes.
We could make onCancelled final, and call a template method from here. If subclasses need to do more stuff when cancelled (ie: delete temp file), that could be written in the template method.

This is similar to what we are doing with the onSuccess() template method.

If you agree I can make the change.