Revert "Remove superfluous OnFailure= directives in target units"

Target units can indeed fail if a dependency fails. As such, it makes
sense to have the OnFailure= in there. systemd just incorrectly warned
about it for a while. See
  https://github.com/systemd/systemd/commit/94d1ddbd7cd15b1073757eb5ae0645c83f0b414c

This reverts commit 11ae5b4c.