fix return value of iter_city_search (closes #457)

weboob/tools/application/formatters/iformatter.py

@@ -97,7 +97,7 @@ class IFormatter(object):
         @param selected_fields  [tuple] fields to display. If None, all fields are selected
         @return  a string of the formatted object
         """
-        assert isinstance(obj, (dict, CapBaseObject, tuple))
+        assert isinstance(obj, (dict, CapBaseObject, tuple)), 'Object is unexpected type "%r"' % obj
 
         if isinstance(obj, dict):
             item = obj