Better error message if workers fall out of sync (#396)

Summary:
This kind of issue should be rare, but the exception that was thrown before ("UnpicklingError: invalid load key") was very opaque, so let's use something a bit clearer.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/396

Differential Revision: D13325600

Pulled By: myleott

fbshipit-source-id: 2e7093752d45d6b04a3d506aca8d5694b72ab638