code: @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public static Completable error(final Callable<? extends Throwable> errorSupplier) {
          ObjectHelper.requireNonNull(errorSupplier, "errorSupplier is null");
          return RxJavaPlugins.onAssembly(new CompletableErrorSupplier(errorSupplier));
      }
docs: Creates a Completable which calls the given error supplier for each subscriber
  and emits its returned Throwable.
  <p>
  <img width="640" height="462" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.error.f.png" alt="">
  <p>
  If the errorSupplier returns null, the child CompletableObservers will receive a
  NullPointerException.
  <dl>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code error} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  @param errorSupplier the error supplier, not null
  @return the new Completable instance
  @throws NullPointerException if errorSupplier is null