Fix Tokenizer.from_pretrained `raise OSError`
`raise` before OSError seems to be forgotten.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2465?src=pr&el=h1) Report
> Merging [#2465](https://codecov.io/gh/huggingface/transformers/pull/2465?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/f599623a99b808e3d5926d89cd13237457b9eeba?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2465/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2465?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2465   +/-   ##
=======================================
  Coverage   73.23%   73.23%           
=======================================
  Files          87       87           
  Lines       15003    15003           
=======================================
  Hits        10988    10988           
  Misses       4015     4015
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2465?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/2465/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `87.56% <ø> (ø)` | :arrow_up: |

------

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