Fix caching issue #12536
# What does this PR do?

This PR is a proposed fix to issue #12536 . It does so by simply logging the unfound file instead of raising an error causing program execution, in the special case of non-existent optional vocab files which are handled in the case `local_files_only=True` (`FileNotFoundError`) and the case where  `local_files_only=False` and user is online, but not the case where `local_files_only=False` and user is offlline.

This needs to be reviewed to ensure this is the direction to go to fix this issue, and that this will not be a problem in other cases.

Fixes #12536


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@LysandreJik 
Note: Tests will have to be changed if you want to go this way, I would imagine it's a bit too general of a fix to be honest.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.