cmd/gofmt: Ignore file not found errors.

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>

src/cmd/gofmt/gofmt.go

@@ -143,7 +143,9 @@ func visitFile(path string, f os.FileInfo, err error) error {
 	if err == nil && isGoFile(f) {
 		err = processFile(path, nil, os.Stdout, false)
 	}
-	if err != nil {
+	// Don't complain if a file was deleted in the meantime (i.e.
+	// the directory changed concurrently while running gofmt).
+	if err != nil && !os.IsNotExist(err) {
 		report(err)
 	}
 	return nil