src/main/java/io/reactivex/Single.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<T> onErrorReturn(final Function<Throwable, ? extends T> resumeFunction) {
        ObjectHelper.requireNonNull(resumeFunction, "resumeFunction is null");
        return RxJavaPlugins.onAssembly(new SingleOnErrorReturn<T>(this, resumeFunction, null));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<T> onErrorReturn(final Function<Throwable, ? extends T> resumeFunction) {
        ObjectHelper.requireNonNull(resumeFunction, "resumeFunction is null");
        return RxJavaPlugins.onAssembly(new SingleOnErrorReturn<T>(this, resumeFunction, null));
    }
Instructs a Single to emit an item (returned by a specified function) rather than invoking
{@link SingleObserver#onError onError} if it encounters an error.
<p>
<img width="640" height="451" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.onErrorReturn.png" alt="">
<p>
By default, when a Single encounters an error that prevents it from emitting the expected item to its
subscriber, the Single invokes its subscriber's {@link SingleObserver#onError} method, and then quits
without invoking any more of its subscriber's methods. The {@code onErrorReturn} method changes this
behavior. If you pass a function ({@code resumeFunction}) to a Single's {@code onErrorReturn} method, if
the original Single encounters an error, instead of invoking its subscriber's
{@link SingleObserver#onError} method, it will instead emit the return value of {@code resumeFunction}.
<p>
You can use this to prevent errors from propagating or to supply fallback data should errors be
encountered.
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code onErrorReturn} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param resumeFunction
a function that returns an item that the new Single will emit if the source Single encounters
an error
@return the original Single with appropriately modified behavior
@see <a href="http://reactivex.io/documentation/operators/catch.html">ReactiveX operators documentation: Catch</a>