Function nested_session_fatal_error 

pub fn nested_session_fatal_error(
    app_name: &str,
    active_gateway: Option<&str>,
    target_socket: &str,
) -> Error

Create a nesting-inception fatal error branded with the given app name, naming BOTH endpoints so a refusal is instantly diagnosable: the active session gateway comes from the host daemon’s inception marker, the requested gateway is the socket this client was about to attach to.