reactor-core/src/main/java/reactor/core/publisher/Mono.java
public final <E extends Throwable> Mono<T> onErrorReturn(Class<E> type, T fallbackValue) {
		return onErrorResume(type, throwable -> just(fallbackValue));
	}
public final <E extends Throwable> Mono<T> onErrorReturn(Class<E> type, T fallbackValue) {
		return onErrorResume(type, throwable -> just(fallbackValue));
	}
Simply emit a captured fallback value when an error of the specified type is
observed on this {@link Mono}.
<p>
<img class="marble" src="doc-files/marbles/onErrorReturnForMono.svg" alt="">

@param type the error type to match
@param fallbackValue the value to emit if an error occurs that matches the type
@param <E> the error type

@return a new falling back {@link Mono}