[release-branch.go1.12] os: consistently return PathError from RemoveAll

Fixes #30859
Updates #30491

Change-Id: If4070e5d39d8649643d7e90f6f3eb499642e25ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/164720


Run-TryBot: Baokun Lee <nototon@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
(cherry picked from commit d039e12b)
Reviewed-on: https://go-review.googlesource.com/c/go/+/167739


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Baokun Lee <nototon@gmail.com>