xdg-autostart: downgrade warning for missing executables

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.