fix: Make UnknownTypesError error more readable

[karlkfi]

• Fix regression from switch to GroupVersionKind from GroupKind
• New error string format: G/V/K, G/V/K
• Avoid K.G/V, which looks like but isn't R.G/V

Fixes: https://github.com/kubernetes-sigs/cli-utils/pull/430#issuecomment-966737188

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ash2k, karlkfi, mortent

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