reactor-core/src/main/java/reactor/core/publisher/Mono.java
public final Mono<T> onErrorResume(Function<? super Throwable, ? extends Mono<? extends
			T>> fallback) {
		return onAssembly(new MonoOnErrorResume<>(this, fallback));
	}
public final Mono<T> onErrorResume(Function<? super Throwable, ? extends Mono<? extends
			T>> fallback) {
		return onAssembly(new MonoOnErrorResume<>(this, fallback));
	}
Subscribe to a fallback publisher when any error occurs, using a function to
choose the fallback depending on the error.

<p>
<img class="marble" src="doc-files/marbles/onErrorResumeForMono.svg" alt="">

@param fallback the function to choose the fallback to an alternative {@link Mono}

@return a {@link Mono} falling back upon source onError

@see Flux#onErrorResume