cmd/godoc: better console error message for example error
cmd/godoc: better console error message for example error

(per r's suggestion)

R=r
CC=golang-dev
https://codereview.appspot.com/7376045

gnanderson/go,apparentlymart/golang,dsriram/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,gnanderson/go,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,tv42/old-go,gnanderson/go,apparentlymart/golang,tv42/old-go,dsriram/golang,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,tv42/old-go