Throw exception is [TopicResponseCache] not applied to TopicController

The `[TopicResponseCache]` will only work with the `TopicController`. Initially, this was validated without an exception since it was intended to be registered as a global filter. In the end, however, it made more sense to register it as an attribute filter on the `TopicController` itself, thus automatically registering it with all derived controllers. Given that, it should throw an explicit exception if a developer attempts to add it to any other type of controller. This maintains consistency with the `[ValidateTopic]` attribute, which performs the same validation.

Author: JeremyCaney

OnTopic.AspNetCore.Mvc/_filters/TopicResponseCacheAttribute.cs

@@ -65,7 +65,9 @@ public override void OnActionExecuting(ActionExecutingContext context) {
       var controller            = context.Controller as TopicController;
 
       if (controller is null) {
-        return;
+        throw new InvalidOperationException(
+          $"The {nameof(TopicResponseCacheAttribute)} can only be applied to a controller deriving from {nameof(TopicController)}."
+        );
       }
 
       /*------------------------------------------------------------------------------------------------------------------------