Improved Error message when loading config/model with .from_pretrained()
Given the previous error message, it can be quite time-consuming to find out that the only problem was that the /path/to/model/dir was incorrect :D 
Thanks to @mariamabarham for pointing this out :-) <|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3247?src=pr&el=h1) Report
> Merging [#3247](https://codecov.io/gh/huggingface/transformers/pull/3247?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/dc848c29944265e04f1473cd0312eeffc1842276&el=desc) will **decrease** coverage by `0.36%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3247/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3247?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3247      +/-   ##
==========================================
- Coverage   78.32%   77.95%   -0.37%     
==========================================
  Files          98       98              
  Lines       16665    16665              
==========================================
- Hits        13053    12992      -61     
- Misses       3612     3673      +61     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3247?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3247/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3V0aWxzLnB5) | `96.82% <ø> (ø)` | |
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/3247/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `28.89% <0.00%> (-27.60%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3247/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `93.84% <0.00%> (+0.13%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3247/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `86.40% <0.00%> (+4.11%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3247?src=pr&el=footer). Last update [dc848c2...cd5998e](https://codecov.io/gh/huggingface/transformers/pull/3247?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
