refactor: sentinel error for running in out of cluster mode

[prometherion]

Closes #594.

With the use of the sentinel error, I can skip the labeling of pods required by the latest changes introduced with #555.

2022-06-19T14:31:53.596+0200    INFO    controllers.TLS skipping annotation of Pods for cert-manager    {"Request.Namespace": "capsule-system", "Request.Name": "capsule-tls", "error": "cannot retrieve the leader Pod, probably running in out of the cluster mode"}

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