Produce a 503 response when there no more worker threads to handle bl…

…ocking requests

(cherry picked from commit dec72dc)

extensions/vertx-http/runtime/src/main/java/io/quarkus/vertx/http/runtime/QuarkusErrorHandler.java

@@ -10,6 +10,7 @@
 import java.util.List;
 import java.util.Optional;
 import java.util.UUID;
+import java.util.concurrent.RejectedExecutionException;
 import java.util.concurrent.atomic.AtomicLong;
 import java.util.function.Consumer;
 
@@ -93,6 +94,12 @@ public void accept(Throwable throwable) {
                 event.response().end();
                 return;
             }
+
+            if (event.failure() instanceof RejectedExecutionException) {
+                // No more worker threads - return a 503
+                event.response().setStatusCode(HttpResponseStatus.SERVICE_UNAVAILABLE.code()).end();
+                return;
+            }
         } catch (IllegalStateException e) {
             //ignore this if the response is already started
             if (!event.response().ended()) {