[k8s] Add debug message for timeout

[romilbhardwaj]

Adds a debugging hint to run kubectl when get_nodes or get_pods fails. This is usually caused by network issues, and running kubectl for debugging will assure the user the issue is related to k8s reachability, not SkyPilot.

Tested (run the relevant ones):

• Code formatting: bash format.sh=

[Michaelvll]

Adding the command looks good to me. Just wondering if we can somehow print the error message directly by calling the kubernetes API in our code when the timeout happens?

[romilbhardwaj]

Thanks! I think it might be better to let the user run it themselves since it may take a long time (e.g., in the event of a network call timeout, default timeout is 60s, retried 3x) and may delay our response.