[RFE] Provide further details when API server connectivity check fails

## Current situation

Currently, the outcome of the `check-apiserver-connectivity` command is mainly `succeeded` or `failed`:

```
$ kubectl aks check-apiserver-connectivity --node aks-agentpool-27170680-vmss000000
Running...

Connectivity check: succeeded
$ echo $?
0
```

```
$ kubectl aks check-apiserver-connectivity --node aks-agentpool-27170680-vmss000000
Running...

Connectivity check: failed with returned value X:
$ echo $?
X
```

However, in case of failure, the `stderr` could not be enough to understand what is the issue.

## Impact

Output could not provide enough details to the user to understand where to start investigation.

## Ideal future situation

When there's a failure, the outcome of `check-apiserver-connectivity` should be enough to understand, at least, when there is a problem with the DNS and when the API sever is actually unreachable.

## Implementation options

Before proceeding with this implementation, find a way to simulate an issue on the DNS and verify what is the `stderr` of the command we are using to verify connectivity (currently `kubectl --kubeconfig /var/lib/kubelet/kubeconfig version`).