Unfortunately Github sometimes has hiccups like this.  They used to report 503 but now apparently return 404s.  The other platforms appear to have built.  Basically there's a non zero chance if you hit a url of it not responding.  We've been working on making our tools hit github less and to have them retry after a few seconds.  I've retriggered this job, I expect it to pass the next time it runs.  We've talked about making the setup stages not email failures if the setup doesn't complete, before the actual tests are run.  I realized it wasn't tickted yet so I opened it at https://github.com/ros-infrastructure/buildfarm/issues/116