Make Unknown Functions Reply to Client

[pseudotronics]

Changed the default case in handle_req_fc to respond instead of silently erroring. This should be done to avoid the connection timing out.

Most clients expect a response even when the function is unsupported.

[debevv]

Right, this was probably made when the calling function was supposed to call send_exception_msg() with the returned error. Shame that tests didn't catch this.
Thank you