src/main/java/io/reactivex/Single.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Single<T> error(final Callable<? extends Throwable> errorSupplier) {
        ObjectHelper.requireNonNull(errorSupplier, "errorSupplier is null");
        return RxJavaPlugins.onAssembly(new SingleError<T>(errorSupplier));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Single<T> error(final Callable<? extends Throwable> errorSupplier) {
        ObjectHelper.requireNonNull(errorSupplier, "errorSupplier is null");
        return RxJavaPlugins.onAssembly(new SingleError<T>(errorSupplier));
    }
Signals a Throwable returned by the callback function for each individual SingleObserver.
<p>
<img width="640" height="283" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.error.c.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code error} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param <T> the value type
@param errorSupplier the callable that is called for each individual SingleObserver and
returns a Throwable instance to be emitted.
@return the new Single instance