misc/emacs: gofmt: don't clobber the current buffer on failure

misc/emacs: gofmt: don't clobber the current buffer on failure

Change M-x gofmt to display errors in a new buffer instead of
clobbering the current buffer.

Add gofmt-before-save, which runs gofmt when in go-mode.  This
can be used with before-save-hook.  Add to your .emacs:
  (add-hook 'before-save-hook 'gofmt-before-save)

R=rsc, aclements, amdragon
CC=golang-dev
https://golang.org/cl/4276059