src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> concatMapDelayError(Function<? super T, ? extends ObservableSource<? extends R>> mapper) {
        return concatMapDelayError(mapper, bufferSize(), true);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> concatMapDelayError(Function<? super T, ? extends ObservableSource<? extends R>> mapper) {
        return concatMapDelayError(mapper, bufferSize(), true);
    }
Maps each of the items into an ObservableSource, subscribes to them one after the other,
one at a time and emits their values in order
while delaying any error from either this or any of the inner ObservableSources
till all of them terminate.
<p>
<img width="640" height="347" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/concatMapDelayError.o.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code concatMapDelayError} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param <R> the result value type
@param mapper the function that maps the items of this ObservableSource into the inner ObservableSources.
@return the new ObservableSource instance with the concatenation behavior