That should not be the way to enable it. The web.config from Sitefinity contains a lot more information then when you create a default one in Visual Studio.
You don't have FTP access to the Sitefinity directory? It seems to me that you should have at least some kind of access to your files, e.g. when you want to upload new themes or templates.
Try to find a way to edit the web.config on the remote server to change the error mode.
For as far I can see, you are doing it in the right way and therefore it should be good to see the actual error.