Add a guardrail for etcd-snapshot

[manuelbuil]

Is your feature request related to a problem? Please describe.

Problem for careless users like me which might try etcd-snapshot when there is no etcd datastore

Describe the solution you'd like

A useful error that makes me aware of the real problem

Describe alternatives you've considered

Additional context

[VestigeJ]

Old behavior visible on v1.29.10+k3s1 - original fix PR was added in November which is why there won't be an issue on v1.32 because the branch didn't exist yet.

$ k3s etcd-snapshot save

WARN[0000] Unknown flag --node-ip found in config.yaml, skipping
WARN[0000] Unknown flag --write-kubeconfig-mode found in config.yaml, skipping
WARN[0000] Unknown flag --token found in config.yaml, skipping
FATA[0000] open /home/ec2-user/.rancher/k3s/server/token: no such file or directory

$ sudo /usr/local/bin/k3s etcd-snapshot save

WARN[0000] Unknown flag --node-ip found in config.yaml, skipping
WARN[0000] Unknown flag --write-kubeconfig-mode found in config.yaml, skipping
WARN[0000] Unknown flag --token found in config.yaml, skipping
FATA[0000] see server log for details: Unauthorized

$ k3s -v

k3s version v1.29.10+k3s1 (ae4df311)
go version go1.22.8

[VestigeJ]

Latest behavior on v1.32

$ k3s etcd-snapshot save

WARN[0000] Unknown flag --node-ip found in config.yaml, skipping
WARN[0000] Unknown flag --write-kubeconfig-mode found in config.yaml, skipping
WARN[0000] Unknown flag --token found in config.yaml, skipping
FATA[0000] Error: open /home/ec2-user/.rancher/k3s/server/token: no such file or directory

$ sudo /usr/local/bin/k3s etcd-snapshot save

WARN[0000] Unknown flag --node-ip found in config.yaml, skipping
WARN[0000] Unknown flag --write-kubeconfig-mode found in config.yaml, skipping
WARN[0000] Unknown flag --token found in config.yaml, skipping
FATA[0000] Error: see server log for details: etcd datastore disabled