API change. Needs further reflection.

««« original CL description
path/filepath: Simplify Walk interface

The last argument of filepath.Walk was removed, and the Visitor
interface now contains an Error method that is called on errors.

Fixes #2237.

R=golang-dev, gri, r
CC=golang-dev
https://golang.org/cl/4964067

»»»

R=r
CC=golang-dev
https://golang.org/cl/4974065