code: @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public final Throwable blockingGet() {
          BlockingMultiObserver<Void> observer = new BlockingMultiObserver<Void>();
          subscribe(observer);
          return observer.blockingGetError();
      }
docs: Subscribes to this Completable instance and blocks until it terminates, then returns null or
  the emitted exception if any.
  <p>
  <img width="640" height="435" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.blockingGet.png" alt="">
  <dl>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code blockingGet} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  @return the throwable if this terminated with an error, null otherwise
  @throws RuntimeException that wraps an InterruptedException if the wait is interrupted