Merge pull request #60585 from alexbrand/kubeadm-external-etcd-preflight

Automatic merge from submit-queue (batch tested with PRs 60585, 62398, 62258, 62042). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

kubeadm: surface external etcd preflight validation errors

**What this PR does / why we need it**: 
This PR fixes a bug where errors that could occur while running preflight against an external etcd cluster were not being surfaced to the user.

**Which issue(s) this PR fixes** :
Fixes https://github.com/kubernetes/kubeadm/issues/719

**Release note**:

```release-note
kubeadm: surface external etcd preflight validation errors
```

cmd/kubeadm/app/preflight/checks.go

@@ -848,13 +848,13 @@ func getEtcdVersionResponse(client *http.Client, url string, target interface{})
 			r, err := client.Get(url)
 			if err != nil {
 				loopCount--
-				return false, nil
+				return false, err
 			}
 			defer r.Body.Close()
 
 			if r != nil && r.StatusCode >= 500 && r.StatusCode <= 599 {
 				loopCount--
-				return false, nil
+				return false, fmt.Errorf("server responded with non-successful status: %s", r.Status)
 			}
 			return true, json.NewDecoder(r.Body).Decode(target)