reactor-core/src/main/java/reactor/core/publisher/Mono.java
public final Mono<T> onErrorMap(Function<? super Throwable, ? extends Throwable> mapper) {
		return onErrorResume(e -> Mono.error(mapper.apply(e)));
	}
public final Mono<T> onErrorMap(Function<? super Throwable, ? extends Throwable> mapper) {
		return onErrorResume(e -> Mono.error(mapper.apply(e)));
	}
Transform any error emitted by this {@link Mono} by synchronously applying a function to it.
<p>
<img class="marble" src="doc-files/marbles/onErrorMapForMono.svg" alt="">

@param mapper the error transforming {@link Function}

@return a {@link Mono} that transforms source errors to other errors