mantle/kola: detect when systemd-generators fail

[dustymabe]

For example, this ugly failure has been hanging around in our console logs for some time:

[    5.733962] ln:
[    5.733968] failed to create symbolic link '/run/udev/rules.d/80-coreos-boot-disk.rules'
[    5.734511] : File exists
[    5.737931] systemd[1023]: /usr/lib/systemd/system-generators/coreos-diskful-generator failed with exit status 1.

Let's detect these when they pop up.

[dustymabe]

xref: https://github.com/coreos/fedora-coreos-config/pull/1797

[dustymabe]

Coverting to draft.. We need to hold this until https://github.com/coreos/fedora-coreos-config/pull/1797 makes it to stable.

[dustymabe]

ok https://github.com/coreos/fedora-coreos-config/pull/1797 was rejected, but https://github.com/coreos/fedora-coreos-config/pull/1801 fixed this problem and that was merged in june so we should be good to merge this I believe.