code: @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public final Completable doOnEvent(final Consumer<? super Throwable> onEvent) {
          ObjectHelper.requireNonNull(onEvent, "onEvent is null");
          return RxJavaPlugins.onAssembly(new CompletableDoOnEvent(this, onEvent));
      }
docs: Returns a Completable which calls the given onEvent callback with the (throwable) for an onError
  or (null) for an onComplete signal from this Completable before delivering said signal to the downstream.
  <p>
  <img width="640" height="305" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.doOnEvent.png" alt="">
  <dl>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code doOnEvent} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  @param onEvent the event callback
  @return the new Completable instance
  @throws NullPointerException if onEvent is null