Improve kubeadm apply error logging style

cmd/kubeadm/app/util/error_test.go

@@ -50,3 +50,34 @@ func TestCheckErr(t *testing.T) {
 		}
 	}
 }
+
+func TestFormatErrMsg(t *testing.T) {
+	errMsg1 := "specified version to upgrade to v1.9.0-alpha.3 is equal to or lower than the cluster version v1.10.0-alpha.0.69+638add6ddfb6d2. Downgrades are not supported yet"
+	errMsg2 := "specified version to upgrade to v1.9.0-alpha.3 is higher than the kubeadm version v1.9.0-alpha.1.3121+84178212527295-dirty. Upgrade kubeadm first using the tool you used to install kubeadm"
+
+	testCases := []struct {
+		errs   []error
+		expect string
+	}{
+		{
+			errs: []error{
+				fmt.Errorf(errMsg1),
+				fmt.Errorf(errMsg2),
+			},
+			expect: "\t-" + errMsg1 + "\n" + "\t-" + errMsg2 + "\n",
+		},
+		{
+			errs: []error{
+				fmt.Errorf(errMsg1),
+			},
+			expect: "\t-" + errMsg1 + "\n",
+		},
+	}
+
+	for _, testCase := range testCases {
+		got := FormatErrMsg(testCase.errs)
+		if got != testCase.expect {
+			t.Errorf("FormatErrMsg error, expect: %v, got: %v", testCase.expect, got)
+		}
+	}
+}