I am writing a http_server application with http_sessions in Prolog. A request is handled with a dicontiguous (multiple definitions) predicate app, which must not fail (otherwise a 500 error appears to be given to the client). I need to early-quit the predicate somehow, so that the rest is not ran, but a custom message is shown and it ends without failure:

:- discontiguous app/1.

app(Request) :-
    ensure_path(Request), !, % I want a `!` after I determined the path is valid, so no other `app` is ran

    % grab session id as `Session`
    http_session_id(Session),
    
    % try to get session data, fail otherwise (I check something else in my code, but `http_session_data` should suffice for this example)
    (http_session_data(my_data(Data), Session); show_error, ...), % some syntactic magic here...
    
    % render something if session data has value
    render_some_success_html(Data).

Here is a piece of pseudocode of a non-Prolog language (don’t scold me for comparing languages of different paradigms) that I am trying to do:

function app(request) {
    if (!ensure_path(request)) ...; // try other app(request)
    let session = http_session_id();
    let data = http_session_data('my_data', session);
    if (!data) {
        show_error();
        return true; // this is important
    }
    render_some_success_html(data);
}

I tried to use a !, fail in place of ..., but that does a failure (swipl says goal unexpectedly failed), and I need a true instead.