core: correct handling of "systemctl kill --kill-who=main-fail"

--kill-who=main-fail never worked correctly, due to a copy and paste
mistake in ac5e3a50, where the same item
was listed twice. The mistake was
later noticed, but fixed incorrectly, in
201f0c91.

Let's list all *-fail types correctly, finally.

And while we are at it, add a nice comment and generate a prettier D-Bus
error about this.