gofmt prints an error to stderr when a file is deleted during its
`filepath.Walk()', which can happen in builds that change the tree
concurrently with gofmt running.

Change-Id: Ia1aa4804f6bc2172baf061c093e16fe56a3ee50c
Reviewed-on: https://go-review.googlesource.com/19301Reviewed-by: Robert Griesemer <gri@golang.org>