Register ErrorController with composition root of Host site

With the introduction of the `ErrorController` (e7fe64f), I'm registering the `ErrorController` with the `SampleActivator`, which acts as the composition root for dependency injection. This will allow the `ErrorController` to be properly configured with the `Host` site.

Author: JeremyCaney

OnTopic.AspNetCore.Mvc.Host/SampleActivator.cs

@@ -117,6 +117,8 @@ public object Create(ControllerContext context) {
       return type.Name switch {
         nameof(TopicController) =>
           new TopicController(_topicRepository, _topicMappingService),
+        nameof(ErrorController) =>
+          new ErrorController(_topicRepository, _topicMappingService),
         nameof(SitemapController) =>
           new SitemapController(_topicRepository),
         nameof(RedirectController) =>