[AutoTokenizer] Fix incorrect from pretrained
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Fixes `tests/test_tokenization_auto.py::AutoTokenizerTest::test_tokenizer_identifier_non_existent` on master.
Currently: 

```python
from transformers import AutoTokenizer; AutoTokenizer.from_pretrained('dont_exist')
```
 yields an incorrect error message

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Models:

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Comments:
> Thanks for the hot fix! Will make a nicer one in a couple of hours.

Perfect