misc/emacs: allow godef to work in coverage buffers

misc/emacs: allow godef to work in coverage buffers

Jumps to the same file will use the original buffer, not the
coverage buffer. Making it work for the coverage buffer isn't
worth the trouble, especially because it would break as soon as
you jump to a different file and back.

Use error instead of message so it actually terminates

R=adonovan
CC=golang-dev
https://golang.org/cl/13041043