Add error message for invalid default fields in ModelStorage.copy

trytond/trytond/model/modelstorage.py

@@ -424,8 +424,12 @@ class ModelStorage(Model):
             if (not isinstance(f, fields.Function)
                 or isinstance(f, fields.MultiValue))
             and n not in mptt]
-        assert (
-            {k.split('.', 1)[0] for k in default.keys()} <= set(fields_names))
+
+        def _default_fields():
+            return {k.split('.', 1)[0] for k in default.keys()}
+        assert _default_fields() <= set(fields_names), (
+            f"Invalid default fields {_default_fields() - set(fields_names)} "
+            f"for {cls.__name__}")
         ids = list(map(int, records))
         with without_check_access():
             values = {