Retry downloading update if update file is missing

This change is for https://www.pivotaltracker.com/story/show/65969536

When Tariff runs in multi-server environment one server may have downloaded the update and marked as pending. If other server tries to apply it, it won't find the file. Therefore we try to redownload the update if file is missing.

It would be better to have  a shared file storage like S3 though.