fail hard when github is down
fail hard when github is down

Summary:public
Sometimes github is down, and replies with an error page, which is not a good
substitute for an opam binary. Failing makes docker not cache the result, so
it's easy to try again when github gets better.

Reviewed By: cristianoc

Differential Revision: D3138561

fb-gh-sync-id: 6725c42
fbshipit-source-id: 6725c42

jvillard/infer,blzq/infer,kageiit/infer,ryoon/infer,facebook/infer,ryoon/infer,facebook/infer,ropas/infer,jvillard/infer,kageiit/infer,jsachs/infer,jsachs/infer,ropas/infer,jsachs/infer,kageiit/infer,facebook/infer,jvillard/infer,ryoon/infer,xennygrimmato/infer,ryoon/infer,kageiit/infer,facebook/infer,facebook/infer,ropas/infer,jsachs/infer,jsachs/infer,kageiit/infer,jvillard/infer,kageiit/infer,kageiit/infer,ryoon/infer,xennygrimmato/infer,xennygrimmato/infer,jvillard/infer,xennygrimmato/infer,xennygrimmato/infer,ropas/infer,blzq/infer,ropas/infer,blzq/infer,facebook/infer,blzq/infer,facebook/infer,blzq/infer,ryoon/infer,blzq/infer,jvillard/infer,xennygrimmato/infer,jsachs/infer,xennygrimmato/infer,ropas/infer,jvillard/infer