Moved exceptions into a new _exceptions folder

This doesn't change the namespace, just helps organize the file structure a bit. The `_camelCase` folders are being used to differentiate namespaces from organizational folders.

Author: JeremyCaney

OnTopic/Mapping/README.md

@@ -193,6 +193,6 @@ While the `CachedTopicMappingService` can be useful for particular scenarios, it
 3. If a graph is manually constructed (by e.g. programmatically mapping `Children`) then the first instance of a topic will be cached independent of the parent graph, thus potentially allowing it to be shared between multiple graphs. This can introduce concerns if edge maintenance is important (e.g., one instance should include children, while another does not).
 
 ## Exceptions
-The topic mapping services will throw a [`TopicMappingException`](TopicMappingException.cs) if a foreseeable exception occurs. Specifically, the exceptions expected will be:
-- **[`InvalidTypeException`](InvalidTypeException.cs):** The [`TopicMappingService`](TopicMappingService.cs) throws this exception if the source `Topic`'s `ContentType` maps to a `TopicViewModel` which cannot be located in the supplied `ITypeLookupService`.
-- **[`MappingModelValidationException`](MappingModelValidationException.cs):** The [`ReverseTopicMappingService`](Reverse/ReverseTopicMappingService.cs) throws this exception if the source model has any discrepancies with the target `Topic` which may introduce unexpected data integrity or data loss once that `Topic` is saved.
+The topic mapping services will throw a [`TopicMappingException`](_exceptions/TopicMappingException.cs) if a foreseeable exception occurs. Specifically, the exceptions expected will be:
+- **[`InvalidTypeException`](_exceptions/InvalidTypeException.cs):** The [`TopicMappingService`](TopicMappingService.cs) throws this exception if the source `Topic`'s `ContentType` maps to a `TopicViewModel` which cannot be located in the supplied `ITypeLookupService`.
+- **[`MappingModelValidationException`](_exceptions/MappingModelValidationException.cs):** The [`ReverseTopicMappingService`](Reverse/ReverseTopicMappingService.cs) throws this exception if the source model has any discrepancies with the target `Topic` which may introduce unexpected data integrity or data loss once that `Topic` is saved.