More clear error message in the use-case of #5169
Author: thomwolf
Pull request: yes

Supplying `is_pretokenized=True` to an encoding method means that the sequences are given as a list of words (words being strings).

Make the error message more clear in this case. 

Fix #5169 

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5184?src=pr&el=h1) Report
> Merging [#5184](https://codecov.io/gh/huggingface/transformers/pull/5184?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/d2a7c86dc33d6def6dba44f6ed2b71e8a1644130&el=desc) will **decrease** coverage by `0.03%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5184/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5184?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5184      +/-   ##
==========================================
- Coverage   78.04%   78.00%   -0.04%     
==========================================
  Files         138      138              
  Lines       23766    23767       +1     
==========================================
- Hits        18548    18540       -8     
- Misses       5218     5227       +9     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5184?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5184/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `94.83% <100.00%> (+0.01%)` | :arrow_up: |
| [src/transformers/data/processors/squad.py](https://codecov.io/gh/huggingface/transformers/pull/5184/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvc3F1YWQucHk=) | `28.66% <0.00%> (-5.42%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5184/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `76.42% <0.00%> (-0.39%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5184/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `85.96% <0.00%> (ø)` | |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/5184/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.57% <0.00%> (+1.18%)` | :arrow_up: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5184/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `80.90% <0.00%> (+1.38%)` | :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/5184?src=pr&el=footer). Last update [d2a7c86...3c842db](https://codecov.io/gh/huggingface/transformers/pull/5184?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).