Correct warning message grammar in the error handling block.

[network-charles]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

While working with kwok, I realized that if I input the wrong cluster name when doing maybe kwokctl get clusters --name <cluster-name> a warning message pops up saying "Cluster is not exists". This grammatically doesn't seem right. I updated it to "Cluster does not exist".

Does this PR introduce a user-facing change?

The initial warning output "Cluster is not exists" has been removed and updated to "Cluster does not exist".

[k8s-ci-robot]

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[wzshiming]

/ok-to-test
/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: network-charles, wzshiming

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