src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.FULL)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Flowable<R> concatMapDelayError(Function<? super T, ? extends Publisher<? extends R>> mapper) {
        return concatMapDelayError(mapper, 2, true);
    }
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.FULL)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Flowable<R> concatMapDelayError(Function<? super T, ? extends Publisher<? extends R>> mapper) {
        return concatMapDelayError(mapper, 2, true);
    }
Maps each of the items into a Publisher, 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 Publishers
till all of them terminate.

<dl>
<dt><b>Backpressure:</b></dt>
<dd>The operator honors backpressure from downstream. Both this and the inner {@code Publisher}s are
expected to honor backpressure as well. If the source {@code Publisher} violates the rule, the operator will
signal a {@code MissingBackpressureException}. If any of the inner {@code Publisher}s doesn't honor
backpressure, that <em>may</em> throw an {@code IllegalStateException} when that
{@code Publisher} completes.</dd>
<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 Publisher into the inner Publishers.
@return the new Publisher instance with the concatenation behavior