Automatic merge from submit-queue (batch tested with PRs 57021, 56843, 54983). 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>.

 Optimizing the implementation of the error check for PriorityClass

**What this PR does / why we need it**:

When i create pod(on the bottom) with not exist PriorityClass, the output will be shown as follow:
```
# kubectl apply -f priorityclassname-pod.yaml 
Error from server: error when creating "priorityclassname-pod.yaml": failed to get default priority class not-exist-priorityclassname: priorityclass.scheduling.k8s.io "not-exist-priorityclassname" not found
```

In my eyes, "get default priority class" is not the correct description, so i changed it. The new output will be shown like this:
```
# kubectl apply -f priorityclassname-pod.yaml 
Error from server (NotFound): error when creating "priorityclassname-pod.yaml": priorityclass.scheduling.k8s.io "not-exist-priorityclassname" not found
```

In addition, the 'pc' will never be nil when err is nil, i think this check is not neccessary, so i removed it.

thank you!

Pod template:
```
apiVersion: v1
kind: Pod
metadata:
  name: priorityclassname-pod           
  labels:
    env: priorityclassname-pod    
spec:
  containers:
  - name: was
    image: gcr.io/google_containers/busybox:v1.0
    imagePullPolicy: IfNotPresent
  priorityClassName: not-exist-priorityclassname
```