I'm not sure if I fully understand your question, but I would say this: Since the Web.config is a server-based file, the user shouldn't see anything in the way of errors. In fact, what you have to do, is when you get to the server(s) that you are going to run the web application on, you would encrypt the file at that server(s), so that the key was stored on that server.
I hope that this is clearer,
Robert