os: RemoveAll no longer returns *os.PathError [1.12 backport]

[gopherbot]

@bradfitz requested issue #30491 to be considered for backport to the next 1.12 minor release.

@gopherbot, please backport to Go 1.12.

[gopherbot]

Change https://golang.org/cl/167739 mentions this issue: [release-branch.go1.12] os: consistently return PathError from RemoveAll

[gopherbot]

Closed by merging ff04803 to release-branch.go1.12.