switch-to-configuration-ng: Die less

This is horrible when logind fails (for any reason) and the services
that were stopped already do not get started anymore because stc-ng
died. This replaces the die() with the exit code so regular tooling
should pick up that something failed without the catastrophic events of
services like sshd not running.

pkgs/by-name/sw/switch-to-configuration-ng/src/main.rs

@@ -2376,7 +2376,7 @@ won't take effect until you reboot the system.
     match logind.list_users() {
         Err(err) => {
             eprintln!("Unable to list users with logind: {err}");
-            die();
+            exit_code = 4;
         }
         Ok(users) => {
             for (uid, name, user_dbus_path) in users {