SubConfig: first argument must be a OD

tiramisu/config.py

@@ -42,6 +42,8 @@ class SubConfig(object):
         :type context: `Config`
         """
         # main option description
+        if not isinstance(descr, OptionDescription):
+            raise ValueError(_('descr must be an optiondescription, not {0}').format(type(descr)))
         self._cfgimpl_descr = descr
         # sub option descriptions
         self._cfgimpl_context = context