nt throws an exception
         then the resulting CompletionStage is completed exceptionally with <a href="https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CompletionException.html?is-external=true" title="class or interface in java.util.concurrent"><code>CompletionException</code></a>
         that wraps all the exceptions raised by observers as suppressed exception.
         If no exception is thrown by observers then the resulting CompletionStage is completed normally with the event payloa