Revert "units: drop OnFailure= from .target units"

This reverts commit c7220ca8.

The removal was done as a reaction to the messages from systemd:
initrd-root-fs.target: Requested dependency OnFailure=emergency.target ignored (target units cannot fail).
initrd.target: Requested dependency OnFailure=emergency.target ignored (target units cannot fail).
initrd-root-device.target: Requested dependency OnFailure=emergency.target ignored (target units cannot fail).
initrd-fs.target: Requested dependency OnFailure=emergency.target ignored (target units cannot fail).
local-fs.target: Requested dependency OnFailure=emergency.target ignored (target units cannot fail).
...
But it seems that the messages themselves are wrong, and the units were OK.