In SharePoint 2007 there was no way to redirect to custom error page when you use event receivers, if you try also it redirects to default out of the box blue-white error page
But in SharePoint 2010 there are some properties by using this you can redirect to custom error page
public override void ItemAdding(SPItemEventProperties properties)
{
base.ItemAdding(properties);
string tite = properties.AfterProperties[“title”].ToString();
if(tite.Contains(“RSM”))
{
properties.Cancel = true;
properties.Status = SPEventRecieverStatus.CancelWithRedirectUrl;
Properties.RedirectUrl...