Throw error if path being set by kops set is not present in struct

[bharath-123]

Currently, when using kops set cluster <cluster_name> =, if the path is not present it fails silently. eg:
kops set cluster test-cluster.k8s.local spec.abc.def.ghi=jkl will not throw any error despite the fields not being present in the kops.Cluster struct. This can lead to confusion and mistakes.

This PR explicitly throws an error if a field being set using kops set does not exist in the associated struct.

Fixes: #10647

Depends on: #10690

[k8s-ci-robot]

Hi @bharath-123. Thanks for your PR.

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[bharath-123]

/cc @rifelpet

[rifelpet]

/ok-to-test

[rifelpet]

/lgtm
/approve

Thanks for improving this!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bharath-123, rifelpet

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