logind-session: be more specific about session_kill() errors

When kill_whom == _ALL, there can be two cases that lead to
ESRCH: the session expects no scope at all or the scope is
not active. Let's distinguish the two cases.

src/login/logind-session-dbus.c

@@ -360,7 +360,7 @@ int bus_session_method_kill(sd_bus_message *message, void *userdata, sd_bus_erro
         if (r == 0)
                 return 1; /* Will call us back */
 
-        r = session_kill(s, whom, signo);
+        r = session_kill(s, whom, signo, error);
         if (r < 0)
                 return r;
 

src/login/logind-session.c

@@ -1387,16 +1387,20 @@ SessionState session_get_state(Session *s) {
         return SESSION_ONLINE;
 }
 
-int session_kill(Session *s, KillWhom whom, int signo) {
+int session_kill(Session *s, KillWhom whom, int signo, sd_bus_error *error) {
         assert(s);
 
         switch (whom) {
 
         case KILL_ALL:
-                if (!s->scope)
-                        return -ESRCH;
+                if (!SESSION_CLASS_WANTS_SCOPE(s->class))
+                        return sd_bus_error_setf(error, SD_BUS_ERROR_NOT_SUPPORTED,
+                                                 "Session '%s' has no associated scope", s->id);
 
-                return manager_kill_unit(s->manager, s->scope, KILL_ALL, signo, NULL);
+                if (!s->scope)
+                        return sd_bus_error_set_errnof(error, ESRCH, "Scope for session '%s' not active", s->id);
+
+                return manager_kill_unit(s->manager, s->scope, KILL_ALL, signo, error);
 
         case KILL_LEADER:
                 return pidref_kill(&s->leader, signo);

src/login/logind-session.h

@@ -191,7 +191,7 @@ int session_finalize(Session *s);
 int session_release(Session *s);
 int session_save(Session *s);
 int session_load(Session *s);
-int session_kill(Session *s, KillWhom whom, int signo);
+int session_kill(Session *s, KillWhom whom, int signo, sd_bus_error *error);
 
 SessionState session_get_state(Session *u);