Validate IsNew on Load(topic, version)

The `Load(topic, version)` is intended to load a previous version of the supplied topic. This is a friendly overload which calls `Load(topicId, version, topic)`. If `topic.IsNew`, however, then it will either fail—or, worse, potentially (attempt to) return that version of the root topic (if it exists). This is because an `IsNew` topic has an `Id` of -1, but some persistence stores treat requests to load a topic with an `Id` of -1 as a reference to the root.

Regardless, this is expected to either fail or return incorrect results if the topic `IsNew`. Given that, it's better to validate this upfront and return a clear error message regarding the issue.

This contributes to #77.

Author: JeremyCaney

OnTopic/Repositories/TopicRepository.cs

@@ -223,6 +223,11 @@ protected ContentTypeDescriptorCollection SetContentTypeDescriptors(ContentTypeD
     /// <inheritdoc />
     public override Topic? Load(Topic topic, DateTime version) {
       Contract.Requires(topic, nameof(topic));
+      Contract.Requires<ArgumentException>(
+        !topic.IsNew,
+        $"The version '{version}' of '{topic.GetUniqueKey()}' cannot be loaded. Topics must be saved in order to load " +
+        $"previous versions."
+      );
       return Load(topic.Id, version, topic);
     }