Changed EnvironmentError to RuntimeError in get_from_cache (#2767)
Changed EnvironmentError to RuntimeError in get_from_cache (#2767)

Summary:
# Before submitting

- [x] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
- [x] Did you read the [contributor guideline](https://github.com/pytorch/fairseq/blob/master/CONTRIBUTING.md)?
- [x] Did you make sure to update the docs?
No need I believe
- [x] Did you write any new necessary tests?
No
## What does this PR do?
Fixes https://github.com/pytorch/fairseq/issues/2724

## PR review
Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

## Did you have fun?
Yes! It is not a big PR at all but it allowed me to familiarize with the caching/downloading logic used in fairseq (which is very similar to that used in pytorch/transformers)

Pull Request resolved: https://github.com/pytorch/fairseq/pull/2767

Reviewed By: edunov

Differential Revision: D24456055

Pulled By: myleott

fbshipit-source-id: bc634a9b97f957ecc5a8da57b112ff892e492107

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq