cmd/godoc: better error message for missing index files
cmd/godoc: better error message for missing index files

Fixes issue 5024.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8222045

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