Udskriv fejlmeddelelse på custom error side

Jeg har en custom fejlside, som man bliver smidt overpå hver gang der er opstået en fejl:

<customErrors mode="On" defaultRedirect="Base/ErrorPage.aspx" />

På denne side vil jeg gerne skrive selve fejlen ud til brugeren. Men bruger jeg Server.GetLastError() på siden, så fejler den.

Hvis jeg i min Global.asax skriver:

protected void Application_Error(Object sender, EventArgs e)
{
    Exception ex = Server.GetLastError();
    Session["Error"] = ex;
}

og i min fejlside prøver at udskrive ex.Message, så får jeg fejlen:

Runtime Error
Description: An application error occurred on the server. The current custom error settings for this application prevent the details of the application error from being viewed remotely (for security reasons). It could, however, be viewed by browsers running on the local server machine.

Details: To enable the details of this specific error message to be viewable on remote machines, please create a <customErrors> tag within a "web.config" configuration file located in the root directory of the current web application. This <customErrors> tag should then have its "mode" attribute set to "Off".


<!-- Web.Config Configuration File -->

<configuration>
    <system.web>
        <customErrors mode="Off"/>
    </system.web>
</configuration>


Notes: The current error page you are seeing can be replaced by a custom error page by modifying the "defaultRedirect" attribute of the application's <customErrors> configuration tag to point to a custom error page URL.


<!-- Web.Config Configuration File -->

<configuration>
    <system.web>
        <customErrors mode="RemoteOnly" defaultRedirect="mycustompage.htm"/>
    </system.web>
</configuration>

Hvorfor får jeg ikke lov til at udskrive den fejl? Hvordan løser jeg problemet?