GitLab

Add an Unwrap method to PathError so it works with the errors.Is/As
functions.

Change-Id: Ia6171c0418584f3cd53ee99d97c687941a9e3109
Reviewed-on: https://go-review.googlesource.com/c/go/+/168097

Reviewed-by: Damien Neil <dneil@google.com>