Fix check of _sql_error_messages

trytond/model/modelsql.py

@@ -1092,7 +1092,7 @@ class ModelSQL(ModelStorage):
                     for name, _, error in cls._sql_constraints:
                         if name in exception[0]:
                             cls.raise_user_error(error)
-                    for name, error in cls._sql_error_messages:
+                    for name, error in cls._sql_error_messages.iteritems():
                         if name in exception[0]:
                             cls.raise_user_error(error)
                 raise