ompletionStage</code></a> allowing further pipeline composition on the asynchronous operation.
         Default asynchronous execution facility is container specific.
         If any observer notified by this event 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