systemctl: print wall message only if successful

systemctl would write to the wall even if unsuccessful.

https://bugs.freedesktop.org/show_bug.cgi?id=60393

src/systemctl/systemctl.c

@@ -1841,7 +1841,7 @@ static int start_special(DBusConnection *bus, char **args) {
         }
 
         r = start_unit(bus, args);
-        if (r >= 0)
+        if (r == EXIT_SUCCESS)
                 warn_wall(a);
 
         return r;