ostadmin: Use g_spawn_check_exit_status()
ostadmin: Use g_spawn_check_exit_status()

Just code cleanup.

sujitfulse/ostree,gatispaeglis/ostree,resin-io/ostree,sujitfulse/ostree,leandrosansilva/ostree,leandrosansilva/ostree,GNOME/ostree,sujitfulse/ostree,leandrosansilva/ostree,gatispaeglis/ostree,sujitfulse/ostree,gatispaeglis/ostree,GNOME/ostree,GNOME/ostree,sujitfulse/ostree,gatispaeglis/ostree,resin-io/ostree,GNOME/ostree,gatispaeglis/ostree,resin-io/ostree,resin-io/ostree,GNOME/ostree,gatispaeglis/ostree,leandrosansilva/ostree,resin-io/ostree,leandrosansilva/ostree