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
```
Automatic merge from submit-queue (batch tested with PRs 54761, 54748, 53991, 54485, 46951). 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>.
admission: unify plugin constructors
It's common in Go to return the actual object in constructors, not **one interface**
it implements. This allows us to implement multiple interfaces, but only have
one constructor. As having private types in constructors, we export all plugin structs, of course with private fields.
Note: super interfaces do not work if there are overlapping methods.
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