back off harder when config.sh fails r=aki

seeing if this helps with the load spikes on git.mozilla.org (e.g. bug 990311)

--HG--
branch : production
extra : transplant_source : %B9%F3%00%DF%F3%BFn%86%E2F%84w%29%89%E8%CC%D0%10%E9a