os: add PathError.Unwrap

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/+/168097Reviewed-by: Damien Neil <dneil@google.com>

src/os/error.go

@@ -32,6 +32,8 @@ type PathError struct {
 
 func (e *PathError) Error() string { return e.Op + " " + e.Path + ": " + e.Err.Error() }
 
+func (e *PathError) Unwrap() error { return e.Err }
+
 // Timeout reports whether this error represents a timeout.
 func (e *PathError) Timeout() bool {
 	t, ok := e.Err.(timeout)

src/os/error_test.go

@@ -5,6 +5,7 @@
 package os_test
 
 import (
+	"errors"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -155,3 +156,10 @@ func TestErrPathNUL(t *testing.T) {
 		t.Fatal("Open should have failed")
 	}
 }
+
+func TestPathErrorUnwrap(t *testing.T) {
+	pe := &os.PathError{Err: os.ErrInvalid}
+	if !errors.Is(pe, os.ErrInvalid) {
+		t.Error("errors.Is failed, wanted success")
+	}
+}