Improve error message when the client cannot be created.

extensions/resteasy-reactive/rest-client-reactive/runtime/src/main/java/io/quarkus/rest/client/reactive/runtime/RestClientBuilderImpl.java

@@ -319,7 +319,7 @@ public <T> T build(Class<T> aClass) throws IllegalStateException, RestClientDefi
 
         ArcContainer arcContainer = Arc.container();
         if (arcContainer == null) {
-            throw new IllegalStateException("The Reactive REST Client is not meant to be used outside of Quarkus");
+            throw new IllegalStateException("The Reactive REST Client needs to be built within the context of a Quarkus application with a valid ArC (CDI) context running.");
         }
 
         RestClientListeners.get().forEach(listener -> listener.onNewClient(aClass, this));