While we're at there, also add a message to prompt the user to install
Graphviz if "dot" command is not found.

Fixes #9178.

LGTM=adg, alex.brainman, cookieo9, rsc
R=rsc, adg, bradfitz, alex.brainman, cookieo9, smyrman
CC=golang-codereviews
https://golang.org/cl/180380043