src/main/java/io/reactivex/Maybe.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Maybe<T> onErrorReturn(Function<? super Throwable, ? extends T> valueSupplier) {
        ObjectHelper.requireNonNull(valueSupplier, "valueSupplier is null");
        return RxJavaPlugins.onAssembly(new MaybeOnErrorReturn<T>(this, valueSupplier));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Maybe<T> onErrorReturn(Function<? super Throwable, ? extends T> valueSupplier) {
        ObjectHelper.requireNonNull(valueSupplier, "valueSupplier is null");
        return RxJavaPlugins.onAssembly(new MaybeOnErrorReturn<T>(this, valueSupplier));
    }
Instructs a Maybe to emit an item (returned by a specified function) rather than invoking
{@link MaybeObserver#onError onError} if it encounters an error.
<p>
<img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/onErrorReturn.png" alt="">
<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 valueSupplier
a function that returns a single value that will be emitted as success value
the current Maybe signals an onError event
@return the new Maybe instance
@see <a href="http://reactivex.io/documentation/operators/catch.html">ReactiveX operators documentation: Catch</a>