code: @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public static <T> Observable<T> error(final Throwable exception) {
          ObjectHelper.requireNonNull(exception, "e is null");
          return error(Functions.justCallable(exception));
      }
docs: Returns an Observable that invokes an {@link Observer}'s {@link Observer#onError onError} method when the
  Observer subscribes to it.
  <p>
  <img width="640" height="220" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/error.item.png" alt="">
  <dl>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code error} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  
  @param exception
  the particular Throwable to pass to {@link Observer#onError onError}
  @param <T>
  the type of the items (ostensibly) emitted by the ObservableSource
  @return an Observable that invokes the {@link Observer}'s {@link Observer#onError onError} method when
  the Observer subscribes to it
  @see <a href="http://reactivex.io/documentation/operators/empty-never-throw.html">ReactiveX operators documentation: Throw</a>