Returns the result value when complete, or throws an (unchecked) exception if completed exceptionally. To better conform with the use of common functional forms, if a computation involved in the completion of this CompletableFuture threw an exception, this method throws an (unchecked) CompletionException with the underlying exception as its cause.

Returns:  the result value

Exceptions:

CancellationException		if the computation was cancelled
CompletionException		if this future completed exceptionally or a completion computation threw an exception