Hi Yukiko,
Thanks for using Syncfusion products.
From your query, we understand that you need to display the error message in the dialog. We have analyzed your requirement and ActionFailure client side event can be used to catch / receive the error thrown in server side. But it is not possible in your application since you are using server side events to perform editing actions.
Kindly refer the below knowledge base document to perform server side validation and display error message in dialog.
Note: We have suggested to achieve the above requirement by sending AJAX post. But you are using ascx (user control) page to render Grid. As informed earlier in F139496, it is not possible to call method from ascx page. so we suggest you achieve above requirement using WebMethods
Regards,
Vignesh Natarajan