Give readable error on unknown recipe names

scone/head/menu_reader.py

@@ -362,6 +362,8 @@ class MenuLoader:
         applicable_user: Optional[str],
     ):
         recipe_class = self._head.recipe_loader.get_class(recipe.kind)
+        if recipe_class is None:
+            raise ValueError(f"No recipe class found for {recipe.kind!r}")
 
         fors = fors + tuple(recipe.for_directives)