Added Error pages in preparation for unit testing ErrorController

These have no content, outside of their `Key`. The goal is to determine if the correct `Topic` is returned from the `ErrorController.HttpAsync()` method.

Author: JeremyCaney

OnTopic.AspNetCore.Mvc.Tests/TestDoubles/TestTopicRepository.cs

@@ -72,6 +72,15 @@ private static Topic CreateFakeData() {
       var pageGroup             = new Topic("PageGroup", "PageGroup", rootTopic);
       _                         = new Topic("PageGroupChild", "Page", pageGroup);
 
+      /*------------------------------------------------------------------------------------------------------------------------
+      | Establish error topics
+      \-----------------------------------------------------------------------------------------------------------------------*/
+      var error                 = new Topic("Error", "Page", rootTopic);
+
+      _                         = new Topic("400", "Page", error);
+      _                         = new Topic("405", "Page", error);
+      _                         = new Topic("Unauthorized", "Page", error);
+
       /*------------------------------------------------------------------------------------------------------------------------
       | Define attributes
       \-----------------------------------------------------------------------------------------------------------------------*/