Summary:
This change lets `registerBundle(bundleId, file)` throw an exception
when the file is empty, improving on the current behavior of an
eventual SIGABRT saying "MAP_FAILED: Invalid argument"
Reviewed By: ridiculousfish
Differential Revision: D16451938
fbshipit-source-id: b8b2d0bfed476319c379122fad59a5bf0a8c813b