catalog: update job begin/done messages

These texts have been slightly misleading previously, as they talked
about units, not jobs, but are actually generated for jobs, not units.
This difference matters as units can change state without a job
requesting that.

Also, the message be02cf6855d2428ba40df7e9d022f03d was particularly
wrong, as it claimed the unit failed, while it actually is the start job
that failed, which is a major difference, as jobs can fail without the
unit actually being placed in a failed state. Let's move this message a
bit up, closed to 39f53479d3a045ac8e11786248231fbf (i.e. the message
seen when a start job finished successfully).

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