Look up the Custom Error Pages in .Net.  You should be able to simply add the following to your web.config file <CustomErrors mode="RemoteOnly" default="/urltoyourcustomerrorpage"> tag to your web.config file. 

However, this will only display the error for the appdomain associated with the web.config file (i.e. if it's in a subdirectory of your website, and an error occurs with a page at the root, it won't display the custom error page noted in the customerrors tag.

You can also display different errors based on the http error that occurred.

Frank