bug: Enhance the error message when an admin.kubeconfig already exists, but is corrupt

[apoorvajagtap]

Describe the bug

Recently I was performing a kcp startup, which failed with the following errors:

cannot create the 'admin.kubeconfig` file with an empty token for the shard-admin user

Later, I determined the issue was due to a pre-existing corrupt kubeconfig (did not have any token or context details for shard-admin user) at .kcp/admin.kubeconfig. After cleaning up .kcp, I re-ran go run ./cmd/kcp start, and it progressed well!

Would it be feasible to enhance the error message, when the admin.kubeconfig file already exists but does not have token details of the shard-admin user, rather than just stating that "it couldn't be created with empty token"? Let me know your thoughts!

If this seems like a useful change, I would like to work on it. Thanks!

Steps To Reproduce

1. Remove the context and token details for shard-admin user in the .kcp/admin.kubeconfig.
2. Build and start the kcp: go run ./cmd/kcp start

Expected Behaviour

A clearer message that indicates the missing context & token details from the existing kubeconfig.

Additional Context

No response

[mjudeikis]

using kcp-dev/helm-chart

Error: cannot create the 'admin.kubeconfig' file with an empty token for the shard-admin user

in the kcp volume:

ls -la
total 8
drwxrwxrwx. 1 root  root    48 Feb 14 10:49 .
drwxr-xr-x. 1 root  root   462 Feb 14 10:49 ..
-rw-------. 1 65532 65532   32 Feb 14 10:49 .admin-token-store
-rw-------. 1 65532 65532 3243 Feb 14 10:49 sa.key
root@kcp-control-plane:/var/local-path-provisioner/pvc-cd809aff-299a-4a58-b7a2-350a460aff52_kcp_kcp# cat .admin-token-store 
LYk.bezB4x'x&>j

after deleting the .admin-token-store and restart it worked 🤷

cat .admin-token-store                                                                                                                                                                                                                                                                                                     
qB!EKv0rr

[MikeSpreitzer]

Isn't the corruption of .admin-token-store itself a bug?