Review: ensure executor shutdown in error case.

Signed-off-by: Yufei Cai <yufei.cai@bosch.io>

java/src/main/java/org/eclipse/ditto/client/streaming/ThingSearchSubscription.java

@@ -114,7 +114,7 @@ public void request(final long n) {
     public void cancel() {
         if (!singleThreadedExecutorService.isShutdown() && !singleThreadedExecutorService.isTerminated()) {
             CompletableFuture.runAsync(this::doCancel, singleThreadedExecutorService)
-                    .thenAccept(unused -> singleThreadedExecutorService.shutdownNow());
+                    .whenComplete((result, error) -> singleThreadedExecutorService.shutdownNow());
         }
     }