Its possible you will want to see the real Asp.Net error messages within Sharepoint.
The following method will allow you to see them
1) edit the web.config within your sharepoint installation directory (eg: C:\Inetpub\wwwroot\wss\VirtualDirectories\80)
find the line which says customErrors mode="On" and change it to "Off".You can also set the value to “RemoteOnly” to troubleshoot a production issue.
2) Also you need to enable CallStack, use find to locate the line within the web.config
<
SafeMode MaxControls="200" CallStack="false" DirectFileDependencies="10" TotalFileDependencies="50" AllowPageLevelTrace="false">
and change the CallStack value to true;