src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Observable<T> concatDelayError(ObservableSource<? extends ObservableSource<? extends T>> sources) {
        return concatDelayError(sources, bufferSize(), true);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Observable<T> concatDelayError(ObservableSource<? extends ObservableSource<? extends T>> sources) {
        return concatDelayError(sources, bufferSize(), true);
    }
Concatenates the ObservableSource sequence of ObservableSources into a single sequence by subscribing to each inner ObservableSource,
one after the other, one at a time and delays any errors till the all inner and the outer ObservableSources terminate.
<p>
<img width="640" height="380" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/concatDelayError.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code concatDelayError} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param <T> the common element base type
@param sources the ObservableSource sequence of ObservableSources
@return the new ObservableSource with the concatenating behavior