misc/dashboard/builder: check http status before processing response
misc/dashboard/builder: check http status before processing response

Occasionally GAE will return a 500 error response, don't treat this as a valid JSON body.

R=adg, dsymonds
CC=golang-dev
http://codereview.appspot.com/6775066
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go