reactor-core/src/main/java/reactor/core/publisher/Mono.java
public static <T> Mono<T> error(Supplier<Throwable> errorSupplier) {
		return onAssembly(new MonoErrorSupplied<>(errorSupplier));
	}
public static <T> Mono<T> error(Supplier<Throwable> errorSupplier) {
		return onAssembly(new MonoErrorSupplied<>(errorSupplier));
	}
Create a {@link Mono} that terminates with an error immediately after being
subscribed to. The {@link Throwable} is generated by a {@link Supplier}, invoked
each time there is a subscription and allowing for lazy instantiation.
<p>
<img class="marble" src="doc-files/marbles/errorWithSupplier.svg" alt="">
<p>
@param errorSupplier the error signal {@link Supplier} to invoke for each {@link Subscriber}
@param <T> the reified {@link Subscriber} type

@return a failing {@link Mono}