What type of PR is this?

/kind cleanup

What this PR does / why we need it:

The Error method of the error returned from Request.Watch was "unknown" even the server returned clear message in the Status struct. It was because Request.Watch used the Result's err member directly, which is an unstructured error from the response which the Result object may use if the caller did not return a structured error.

The patch fixes it by calling the Result's Error method instead, which returns the structured error when it's present.

It also removes a duplicate test case and remove the wrong expectation about events.

Testing with the following code:

watcher, err := k8sClient.CoreV1().Secrets(secretMeta.Namespace).Watch(context.TODO(), metav1.SingleObject(*secretMeta))
if err != nil {
	klog.ErrorS(err, "Watch Secret error")
}

Before:

E0722 03:35:57.609475       1 controller.go:302] "Watch Secret error" err="unknown (get secrets)"

After:

E0722 04:14:05.895487       1 controller.go:302] "Watch Secret error" err="secrets \"foo\" is forbidden: User \"system:serviceaccount:kube-system:antrea-agent\" cannot watch resource \"secrets\" in API group \"\" in the namespace \"kube-system\""

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.: