Prevent docker-compose from running if images load failed

[rgaudin]

ATM, we have a dependency between docker-compose and docker-load-images but given we now start docker-compose manually (in the offspot-runtime script) so the it's not blocking which is confusing. offspot/overview#44

We could test that images were imported properly (load script returned 0 and no tarball in folder) in an ExecCondition