Throw TopicNotFoundException in Load(int topicId, DateTime version)

Implemented the new `TopicNotFoundException` (2300e69) in `Load(int topicId, DateTime version)`. This was previously implemented for `Load(int topicId)` and `Load(string topicKey)`.

These implementations previously threw a `NullReferenceException` if a topic wasn't found, which it shouldn't have been doing. Fortunately, it wasn't publicly documented, and no implementers have been catching it, so we're going to treat this as a bug fix, not a breaking change.

Author: JeremyCaney

OnTopic.Data.Sql/SqlTopicRepository.cs

@@ -254,7 +254,7 @@ public override Topic Load(int topicId, DateTime version) {
       /*------------------------------------------------------------------------------------------------------------------------
       | Return objects
       \-----------------------------------------------------------------------------------------------------------------------*/
-      return topic?? throw new NullReferenceException("The specified Topic version could not be loaded");
+      return topic?? throw new TopicNotFoundException(topicId);
 
     }
 

OnTopic.TestDoubles/StubTopicRepository.cs

@@ -97,7 +97,7 @@ public StubTopicRepository() : base() {
       /*------------------------------------------------------------------------------------------------------------------------
       | Return objects
       \-----------------------------------------------------------------------------------------------------------------------*/
-      return topic?? throw new NullReferenceException("The specified Topic version could not be loaded");
+      return topic?? throw new TopicNotFoundException(topicId);
 
     }