Folders: Return 409 Conflict status when folder already exists (#36429)

* Return 409 Conflict when trying to post folder that already exists

* Fix tests

* Update documentation for new error message in folders api

docs/sources/http_api/folder.md

@@ -154,6 +154,7 @@ Status Codes:
 - **400** – Errors (invalid json, missing or invalid fields, etc)
 - **401** – Unauthorized
 - **403** – Access Denied
+- **409** - Folder already exists
 
 ## Update folder