godoc: fix vet warnings for untagged struct literals
godoc: fix vet warnings for untagged struct literals

src/cmd/godoc/filesystem.go:337:10: os.PathError struct literal uses untagged fields
src/cmd/godoc/filesystem.go:355:10: os.PathError struct literal uses untagged fields

R=golang-dev, dsymonds
CC=golang-dev
http://codereview.appspot.com/5820046

wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old