Cli: add the cluster name to the error message

[fstrudel]

• When you ask for a kubeconfig file for a named
  cluster (i..e not the default "kind"), instead of
  getting the (cryptic) message:
  "could not locate any control plane nodes",
  also return the name of the cluster, and a hint
  to what the user should maybe supply (the --name
  option).
• Note: Did not find any existing tests for this file, nor which mock
  libraries we should use to mock the dependencies?

Testing Done: Create a named cluster and tested the get kubeconfig option:

$ make build unit verify
$ ./bin/kind create cluster --name mytest

$ ./bin/kind get kubeconfig
ERROR: could not locate any control plane nodes for cluster named 'kind'. Use the --name option to select a different cluster

$ ./bin/kind get kubeconfig --name mytest
apiVersion: v1
clusters:
- cluster:
    certificate-authority-data: LS0tLS[...]

Bug Number: #2205

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[aojea]

@fstrudel you have to sign the CLA
/ok-to-test
/lgtm

[fstrudel]

/check-cla

[fstrudel]

/retest

[fstrudel]

/check-cla

[neolit123]

/lgtm

[aojea]

/approve
Thanks

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aojea, fstrudel

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