Fixes#43645.
`FailureAction=exit` propagates the exit status of the unit's main
process. When there is none, `emergency_action()` left `m->return_value`
at its initial 0, so the manager exited 0 for a unit that failed —
indistinguishable from an orderly shutdown to whoever supervises it.
Luca Boccassi
bc17a73107