code: public static <U> CompletionStage<U> failedStage(Throwable ex) {
      return new MinimalStage<U>(new AltResult(Objects.requireNonNull(ex)));
    }
docs: Returns a new CompletionStage that is already completed exceptionally with the given exception
  and supports only those methods in interface {@link CompletionStage}.
  
  @param ex the exception
  @param <U> the type of the value
  @return the exceptionally completed CompletionStage
  @since 9