In SharePoint there is not any functionality for displaying our customized error page.Like if for data validation in event/item handler we use
properties.ErrorMessage = “Opps… Not allowed”;
properties.Cancel = true;
And validation message displayed in SharePoint error Page which looks error in application.
But with this Context you can use to redirect to any of your custom or any out of the box Page.
SPUtility.Redirect(Url, SPRedirectFlags.Default, current);
Here current is nothing but the HTTPContext object that u create in the constructor of the Event handler class