generator: Restore graceful exit behaviour if `ostree` karg missing

generator: Restore graceful exit behaviour if `ostree` karg missing

In CoreOS live environments, we do have `/run/ostree` but no `ostree=`
karg; we hackily fool `ostree-prepare-root.service` by bind-mounting
over `/proc/cmdline` so it does the right thing. Presumably, we should
clean this up eventually, but even so we don't want to require PXE users
to add an `ostree=` arg, so we need to tolerate this.

So this assertion would fail there. Restore the behaviour prior to
b9ce0e89 and re-add a more contemporary comment.

Fixes b9ce0e89 ("generator: Exit if there's no `/run/ostree`").