core: make unit_start() return a distinguishable error code in case conditions didn't hold

Ideally we'd even propagate this all the way to the client, by having a
separate JobType enum value for this. But it's hard to add this without
breaking compat, hence for now let's at least internally propagate this
case differently from the case "already on it".

This is then used to call job_finish_and_invalidate() slightly
differently, with the already= parameter false, as in the failed
condition case no message was likely produced so far.