code: @CheckReturnValue
      @BackpressureSupport(BackpressureKind.PASS_THROUGH)
      @SchedulerSupport(SchedulerSupport.NONE)
      public static <T> Flowable<T> error(final Throwable throwable) {
          ObjectHelper.requireNonNull(throwable, "throwable is null");
          return error(Functions.justCallable(throwable));
      }
docs: Returns a Flowable that invokes a {@link Subscriber}'s {@link Subscriber#onError onError} method when the
  Subscriber subscribes to it.
  <p>
  <img width="640" height="190" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/error.png" alt="">
  <dl>
  <dt><b>Backpressure:</b></dt>
  <dd>This source doesn't produce any elements and effectively ignores downstream backpressure.</dd>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code error} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  
  @param throwable
  the particular Throwable to pass to {@link Subscriber#onError onError}
  @param <T>
  the type of the items (ostensibly) emitted by the Publisher
  @return a Flowable that invokes the {@link Subscriber}'s {@link Subscriber#onError onError} method when
  the Subscriber subscribes to it
  @see <a href="http://reactivex.io/documentation/operators/empty-never-throw.html">ReactiveX operators documentation: Throw</a>