grub2: Exit gracefully if there's no system ostree repository
grub2: Exit gracefully if there's no system ostree repository

Apparently there testing systems that literally install *all*
packages.  Having `ostree-grub2` currently causes grub2 to fail
on a non-ostree managed system.  Let's just gracefully exit
if there's no system repository.

https://bugzilla.redhat.com/show_bug.cgi?id=1532668

Closes: #1399
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree