Automatic merge from submit-queue

[Pet Set] Refactor return []error to error

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**Which issue this PR fixes** 

I propose refactor return value from `[]error` to error [here](https://github.com/kubernetes/kubernetes/blob/master/pkg/controller/petset/pet_set.go#L80), for the purpose of simplify code and format error output.

I think we can use `errors.NewAggregate(errorList)` to aggregate `[]error`.