feat: add updated validation message and rename related housing provider -> stock owner

config/locales/en.yml

@@ -117,9 +117,9 @@ en:
       name_missing: "Enter the name of the organisation"
       provider_type_missing: "Select the organisation type"
       housing_provider:
-        blank: "You must choose a housing provider"
-        already_added: "You have already added this housing provider"
-        does_not_own_stock: "Enter an organisation that owns housing stock"
+        blank: "You must choose a stock owner"
+        already_added: "You have already added this stock owner"
+        does_not_own_stock: "You can only add stock owners who own stock, which this organisation does not."
       managing_agent:
         blank: "You must choose a managing agent"
         already_added: "You have already added this managing agent"