fix: change log severity when artifact is not found

[CiprianAnton]

Artifacts can be optional, logging a missing output artifact as error has too high severity, this will be highly visible in the logs requiring attention. Change severity to warning, similar with what is being logged in wait container: https://github.com/argoproj/argo-workflows/blob/master/workflow/executor/executor.go#L316

Fixes #change log severity when artifact is not found

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