fix: proper error message when web ui not enabled

cli/src/main.rs

@@ -432,11 +432,14 @@ fn main() -> Result<()> {
                                 actix_files::Files::new("/", ui_path).index_file("index.html"),
                             );
                         }
-                    } else {
-                        // TODO - 503 error
                     }
 
-                    app
+                    #[actix_web::get("/")]
+                    async fn unavailable_index() -> actix_web::HttpResponse {
+                        actix_web::HttpResponse::ServiceUnavailable().body("Web UI not enabled.")
+                    }
+
+                    return app.service(unavailable_index);
                 });
 
                 let bind_result = server.bind(&bind);