auth: improve pam flow and user status

backend/actions.py

@@ -133,8 +133,15 @@ def disable_user(
                     if play_sound:
                         _play_sound_if_available()
 
-    _run(["sudo", "pkill", "-KILL", "-u", user])
-    steps.append("sessions terminated")
+    try:
+        _run(["sudo", "pkill", "-KILL", "-u", user])
+        steps.append("sessions terminated")
+    except subprocess.CalledProcessError as exc:
+        if exc.returncode == 1:
+            # pkill returns 1 when no matching processes exist; not an error here.
+            steps.append("no sessions to terminate")
+        else:
+            raise
 
     if logged_in:
         _run(["sudo", "shutdown", "now"])