src/main/java/io/reactivex/Maybe.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Maybe<T> error(Callable<? extends Throwable> supplier) {
        ObjectHelper.requireNonNull(supplier, "errorSupplier is null");
        return RxJavaPlugins.onAssembly(new MaybeErrorCallable<T>(supplier));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Maybe<T> error(Callable<? extends Throwable> supplier) {
        ObjectHelper.requireNonNull(supplier, "errorSupplier is null");
        return RxJavaPlugins.onAssembly(new MaybeErrorCallable<T>(supplier));
    }
Returns a Maybe that invokes a {@link MaybeObserver}'s {@link MaybeObserver#onError onError} method when the
MaybeObserver subscribes to it.
<p>
<img width="640" height="190" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/error.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code error} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param supplier
a Callable factory to return a Throwable for each individual MaybeObserver
@param <T>
the type of the items (ostensibly) emitted by the Maybe
@return a Maybe that invokes the {@link MaybeObserver}'s {@link MaybeObserver#onError onError} method when
the MaybeObserver subscribes to it
@see <a href="http://reactivex.io/documentation/operators/empty-never-throw.html">ReactiveX operators documentation: Throw</a>