xdg-autostart: downgrade warning for missing executables

On a system with a shared home directory, I'm getting a bunch of warnings:

systemd-xdg-autostart-generator[76]: Exec binary '/usr/bin/flatpak' does not exist: No such file or directory
systemd-xdg-autostart-generator[76]: /home/zbyszek/.config/autostart/org.signal.Signal.desktop: not generating unit, error parsing Exec= line: No such file or directory
systemd-xdg-autostart-generator[76]: Exec binary '/usr/bin/flatpak' does not exist: No such file or directory
systemd-xdg-autostart-generator[76]: /home/zbyszek/.config/autostart/im.riot.Riot.desktop: not generating unit, error parsing Exec= line: No such file or directory
systemd-xdg-autostart-generator[76]: Exec binary '/usr/libexec/gnome-tweak-tool-lid-inhibitor' does not exist: No such file or directory
systemd-xdg-autostart-generator[76]: /home/zbyszek/.config/autostart/ignore-lid-switch-tweak.desktop: not generating unit, error parsing Exec= line: No such file or directory
systemd-xdg-autostart-generator[76]: Exec binary '/usr/bin/flatpak' does not exist: No such file or directory
systemd-xdg-autostart-generator[76]: /home/zbyszek/.config/autostart/org.telegram.desktop.desktop: not generating unit, error parsing Exec= line: No such file or directory

This isn't really a problem. Let's just print an info message.

(cherry picked from commit ed73914)

src/xdg-autostart-generator/xdg-autostart-service.c

@@ -571,7 +571,10 @@ int xdg_autostart_service_generate_unit(
 
         r = xdg_autostart_format_exec_start(service->exec_string, &exec_start);
         if (r < 0) {
-                log_warning_errno(r, "%s: not generating unit, error parsing Exec= line: %m", service->path);
+                log_full_errno(r == -ENOENT ? LOG_INFO : LOG_WARNING, r,
+                               r == -ENOENT ? "%s: not generating unit, executable specified in Exec= does not exist."
+                                            : "%s: not generating unit, error parsing Exec= line: %m",
+                               service->path);
                 return 0;
         }