ASP.NET web forms mostly eliminate the need for any kind of error page. Instead you should present the error in the page with the form entry fields. Usually the validator would be located right next to the entry field it is validating. Thus, when you submit with invalid data the user sees this next to the invalid field. Only when you get a valid entry for everything would you do some action and redirect the user to a success notice page.
When you run validation logic and set the IsValid property of a validator to true, the page framework will automatically configure the validator so it will show up when the page is rendered and emitted to the user.