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

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 ac5e3a505e49c80b56c971a8fc13bacac961640d, where the same item
was listed twice. The mistake was
later noticed, but fixed incorrectly, in
201f0c916d8f65ad2595a651b1371fcd39a4cf55.

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.