cmd/gofmt, bufio, image: Consistency and error handling.

cmd/gofmt: Add error handling for ioutil.WriteFile.
bufio: Consistency, rename e to err.
image: Consistency, fix comment for asReader.

R=golang-dev, dave, minux.ma, adg
CC=golang-dev
https://codereview.appspot.com/7029056

Committer: Andrew Gerrand <adg@golang.org>