reactor-core/src/main/java/reactor/core/publisher/Flux.java
public final <E extends Throwable> Flux<T> onErrorMap(Class<E> type,
			Function<? super E, ? extends Throwable> mapper) {
		@SuppressWarnings("unchecked")
		Function<Throwable, Throwable> handler = (Function<Throwable, Throwable>)mapper;
		return onErrorMap(type::isInstance, handler);
	}
public final <E extends Throwable> Flux<T> onErrorMap(Class<E> type,
			Function<? super E, ? extends Throwable> mapper) {
		@SuppressWarnings("unchecked")
		Function<Throwable, Throwable> handler = (Function<Throwable, Throwable>)mapper;
		return onErrorMap(type::isInstance, handler);
	}
Transform an error emitted by this {@link Flux} by synchronously applying a function
to it if the error matches the given type. Otherwise let the error pass through.
<p>
<img class="marble" src="doc-files/marbles/onErrorMapWithClassPredicateForFlux.svg" alt="">

@param type the class of the exception type to react to
@param mapper the error transforming {@link Function}
@param <E> the error type

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