logind: log a more accurate error when we failed at session creation

We used to log the following error:

  "Start job for unit user-1000.slice failed with 'canceled'"

which can be really misleading if the actual job failed at *stopping* a unit.

Indeed "Start" was hard coded but it was wrong since we can also fail with stop
jobs which are enqueued when a session is stopped.

(cherry picked from commit b39648ed)