code: public boolean completeExceptionally(Throwable ex) {
          if (ex == null) throw new NullPointerException();
          boolean triggered = internalComplete(new AltResult(ex));
          postComplete();
          return triggered;
      }
docs: If not already completed, causes invocations of {@link #get()}
  and related methods to throw the given exception.
  
  @param ex the exception
  @return {@code true} if this invocation caused this CompletableFuture
  to transition to a completed state, else {@code false}