reactor-core/src/main/java/reactor/core/publisher/Mono.java
public static Mono<Void> whenDelayError(final Iterable<? extends Publisher<?>> sources) {
		return onAssembly(new MonoWhen(true, sources));
	}
public static Mono<Void> whenDelayError(final Iterable<? extends Publisher<?>> sources) {
		return onAssembly(new MonoWhen(true, sources));
	}
Aggregate given publishers into a new {@literal Mono} that will be
fulfilled when all of the given {@literal sources} have completed. If any Publisher
terminates without value, the returned sequence will be terminated immediately and
pending results cancelled. Errors from the sources are delayed.
If several Publishers error, the exceptions are combined (as suppressed exceptions on a root exception).

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

@param sources The sources to use.

@return a {@link Mono}.