Issue: Some languages in wikipedia dataset are not loading

Comment:
Ok, thanks for clarifying, that makes sense. I will time those examples later today and post back here.

Also, it seems that not all dumps should use the same date. For instance, I was checking the Spanish dump doing the following:
```
data = nlp.load_dataset('wikipedia', '20200501.es', beam_runner='DirectRunner', split='train')
```

I got the error below because this URL does not exist: https://dumps.wikimedia.org/eswiki/20200501/dumpstatus.json. So I checked the actual available dates here https://dumps.wikimedia.org/eswiki/ and there is no 20200501. If one tries for a date available in the link, then the nlp library does not allow such a request because is not in the list of expected datasets.

```
Downloading and preparing dataset wikipedia/20200501.es (download: Unknown size, generated: Unknown size, post-processed: Unknown sizetotal: Unknown size) to /home/gaguilar/.cache/huggingface/datasets/wikipedia/20200501.es/1.0.0/7be7f4324255faf70687be8692de57cf79197afdc33ff08d6a04ed602df32d50...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/gaguilar/.conda/envs/pytorch/lib/python3.8/site-packages/nlp/load.py", line 548, in load_dataset
    builder_instance.download_and_prepare(
  File "/home/gaguilar/.conda/envs/pytorch/lib/python3.8/site-packages/nlp/builder.py", line 462, in download_and_prepare
    self._download_and_prepare(
  File "/home/gaguilar/.conda/envs/pytorch/lib/python3.8/site-packages/nlp/builder.py", line 965, in _download_and_prepare
    super(BeamBasedBuilder, self)._download_and_prepare(
  File "/home/gaguilar/.conda/envs/pytorch/lib/python3.8/site-packages/nlp/builder.py", line 518, in _download_and_prepare
    split_generators = self._split_generators(dl_manager, **split_generators_kwargs)
  File "/home/gaguilar/.conda/envs/pytorch/lib/python3.8/site-packages/nlp/datasets/wikipedia/7be7f4324255faf70687be8692de57cf79197afdc33ff08d6a04ed602df32d50/wikipedia.py", line 422, in _split_generators
    downloaded_files = dl_manager.download_and_extract({"info": info_url})
  File "/home/gaguilar/.conda/envs/pytorch/lib/python3.8/site-packages/nlp/utils/download_manager.py", line 220, in download_and_extract
    return self.extract(self.download(url_or_urls))
  File "/home/gaguilar/.conda/envs/pytorch/lib/python3.8/site-packages/nlp/utils/download_manager.py", line 155, in download
    downloaded_path_or_paths = map_nested(
  File "/home/gaguilar/.conda/envs/pytorch/lib/python3.8/site-packages/nlp/utils/py_utils.py", line 163, in map_nested
    return {
  File "/home/gaguilar/.conda/envs/pytorch/lib/python3.8/site-packages/nlp/utils/py_utils.py", line 164, in <dictcomp>
    k: map_nested(
  File "/home/gaguilar/.conda/envs/pytorch/lib/python3.8/site-packages/nlp/utils/py_utils.py", line 191, in map_nested
    return function(data_struct)
  File "/home/gaguilar/.conda/envs/pytorch/lib/python3.8/site-packages/nlp/utils/download_manager.py", line 156, in <lambda>
    lambda url: cached_path(url, download_config=self._download_config,), url_or_urls,
  File "/home/gaguilar/.conda/envs/pytorch/lib/python3.8/site-packages/nlp/utils/file_utils.py", line 191, in cached_path
    output_path = get_from_cache(
  File "/home/gaguilar/.conda/envs/pytorch/lib/python3.8/site-packages/nlp/utils/file_utils.py", line 356, in get_from_cache
    raise ConnectionError("Couldn't reach {}".format(url))
ConnectionError: Couldn't reach https://dumps.wikimedia.org/eswiki/20200501/dumpstatus.json
```