store the ValidationFailed message in workflow history, not the ValidationFailed object itself.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@4228 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Type/ZopePatch.py

@@ -553,7 +553,7 @@ def DCWorkflowDefinition_executeTransition(self, ob, tdef=None, kwargs=None):
             script(sci)  # May throw an exception.
         except ValidationFailed, validation_exc:
             before_script_success = 0
-            before_script_error_message = validation_exc
+            before_script_error_message = str(validation_exc)
         except ObjectMoved, moved_exc:
             ob = moved_exc.getNewObject()
             # Re-raise after transition