systemd: don't require lang file to exist
    
    commit fbb922cfcf4e389b0aa416d9a81c45f8f938f104 made sure
    to source /etc/default/locale on debian so that GDM gets
    started with the correct locale.
    
    Unfortunately, now if that file doesn't exist (which is entirely
    legitimate) then GDM won't start.
    
    This commit makes sure to still start GDM, even if the file couldn't
    be sourced.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=745868