run.bash: set -e in new codewalk block

Otherwise we won't fail if something goes wrong.
This shell programming stuff is tricky.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5905062

src/run.bash

@@ -76,6 +76,7 @@ make clean
 
 (xcd ../doc/codewalk
 # TODO: test these too.
+set -e
 go build pig.go
 go build urlpoll.go
 rm -f pig urlpoll