I tested failing xflock4 by Xubuntu 18.04.1 live session (that has xfce4-power-manager 1.6.1-0ubuntu1). I killed light-locker and tested that xflock4 fails returning 1. Power manager is set to lock screen on suspend. Then I run 'xfce4-session-logout --suspend'. No lock, no error dialog. Same thing, if I suspend from the logout dialog.
I tested failing xflock4 by Xubuntu 18.04.1 live session (that has xfce4-power-manager 1.6.1-0ubuntu1). I killed light-locker and tested that xflock4 fails returning 1. Power manager is set to lock screen on suspend. Then I run 'xfce4- session- logout --suspend'. No lock, no error dialog. Same thing, if I suspend from the logout dialog.