CSI: tolerate missing plugins on job delete

[tgross]

Fixes #11758

If a plugin job fails before successfully fingerprinting the plugins,
the plugin will not exist when we try to delete the job. Tolerate
missing plugins.

---

$ nomad job stop -purge badplugin
==> 2022-02-23T16:02:06-05:00: Monitoring evaluation "c3b163b5"
    2022-02-23T16:02:06-05:00: Evaluation triggered by job "badplugin"
==> 2022-02-23T16:02:07-05:00: Monitoring evaluation "c3b163b5"
    2022-02-23T16:02:07-05:00: Evaluation status changed: "pending" -> "complete"
==> 2022-02-23T16:02:07-05:00: Evaluation "c3b163b5" finished with status "complete"

$ nomad job status badplugin
No job(s) with prefix or id "badplugin" found

[jrasell]

LGTM!

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