Issue: Some languages in wikipedia dataset are not loading

Comment:
I was trying to download dataset for `es` language, however I am getting the following error:
```
dataset = load_dataset('wikipedia', language='es', date="20210320", beam_runner='DirectRunner') 
```

```
Downloading and preparing dataset wikipedia/20210320.es (download: Unknown size, generated: Unknown size, post-processed: Unknown size, total: Unknown size) to /scratch/user_name/datasets/wikipedia/20210320.es-date=20210320,language=es/0.0.0/2fe8db1405aef67dff9fcc51e133e1f9c5b0106f9d9e9638188176d278fd5ff1...
Traceback (most recent call last):
  File "apache_beam/runners/common.py", line 1233, in apache_beam.runners.common.DoFnRunner.process
  File "apache_beam/runners/common.py", line 581, in apache_beam.runners.common.SimpleInvoker.invoke_process
  File "apache_beam/runners/common.py", line 1368, in apache_beam.runners.common._OutputProcessor.process_outputs
  File "/scratch/user_name/modules/datasets_modules/datasets/wikipedia/2fe8db1405aef67dff9fcc51e133e1f9c5b0106f9d9e9638188176d278fd5ff1/wikipedia.py", line 492, in _clean_content
    text = _parse_and_clean_wikicode(raw_content, parser=mwparserfromhell)
  File "/scratch/user_name/modules/datasets_modules/datasets/wikipedia/2fe8db1405aef67dff9fcc51e133e1f9c5b0106f9d9e9638188176d278fd5ff1/wikipedia.py", line 548, in _parse_and_clean_wikicode
    section_text.append(section.strip_code().strip())
  File "/opt/conda/lib/python3.7/site-packages/mwparserfromhell/wikicode.py", line 639, in strip_code
    stripped = node.__strip__(**kwargs)
  File "/opt/conda/lib/python3.7/site-packages/mwparserfromhell/nodes/html_entity.py", line 60, in __strip__
    return self.normalize()
  File "/opt/conda/lib/python3.7/site-packages/mwparserfromhell/nodes/html_entity.py", line 150, in normalize
    return chr(htmlentities.name2codepoint[self.value])
KeyError: '000nbsp'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "download_dataset_all.py", line 8, in <module>
    dataset = load_dataset('wikipedia', language=language, date="20210320", beam_runner='DirectRunner') 
  File "/opt/conda/lib/python3.7/site-packages/datasets/load.py", line 748, in load_dataset
    use_auth_token=use_auth_token,
  File "/opt/conda/lib/python3.7/site-packages/datasets/builder.py", line 575, in download_and_prepare
    dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs
  File "/opt/conda/lib/python3.7/site-packages/datasets/builder.py", line 1152, in _download_and_prepare
    pipeline_results = pipeline.run()
  File "/opt/conda/lib/python3.7/site-packages/apache_beam/pipeline.py", line 564, in run
    return self.runner.run_pipeline(self, self._options)
  File "/opt/conda/lib/python3.7/site-packages/apache_beam/runners/direct/direct_runner.py", line 131, in run_pipeline
    return runner.run_pipeline(pipeline, options)
  File "/opt/conda/lib/python3.7/site-packages/apache_beam/runners/portability/fn_api_runner/fn_runner.py", line 190, in run_pipeline
    pipeline.to_runner_api(default_environment=self._default_environment))
  File "/opt/conda/lib/python3.7/site-packages/apache_beam/runners/portability/fn_api_runner/fn_runner.py", line 200, in run_via_runner_api
    return self.run_stages(stage_context, stages)
  File "/opt/conda/lib/python3.7/site-packages/apache_beam/runners/portability/fn_api_runner/fn_runner.py", line 366, in run_stages
    bundle_context_manager,
  File "/opt/conda/lib/python3.7/site-packages/apache_beam/runners/portability/fn_api_runner/fn_runner.py", line 562, in _run_stage
    bundle_manager)
  File "/opt/conda/lib/python3.7/site-packages/apache_beam/runners/portability/fn_api_runner/fn_runner.py", line 602, in _run_bundle
    data_input, data_output, input_timers, expected_timer_output)
  File "/opt/conda/lib/python3.7/site-packages/apache_beam/runners/portability/fn_api_runner/fn_runner.py", line 903, in process_bundle
    result_future = self._worker_handler.control_conn.push(process_bundle_req)
  File "/opt/conda/lib/python3.7/site-packages/apache_beam/runners/portability/fn_api_runner/worker_handlers.py", line 378, in push
    response = self.worker.do_instruction(request)
  File "/opt/conda/lib/python3.7/site-packages/apache_beam/runners/worker/sdk_worker.py", line 610, in do_instruction
    getattr(request, request_type), request.instruction_id)
  File "/opt/conda/lib/python3.7/site-packages/apache_beam/runners/worker/sdk_worker.py", line 647, in process_bundle
    bundle_processor.process_bundle(instruction_id))
  File "/opt/conda/lib/python3.7/site-packages/apache_beam/runners/worker/bundle_processor.py", line 1001, in process_bundle
    element.data)
  File "/opt/conda/lib/python3.7/site-packages/apache_beam/runners/worker/bundle_processor.py", line 229, in process_encoded
    self.output(decoded_value)
  File "apache_beam/runners/worker/operations.py", line 356, in apache_beam.runners.worker.operations.Operation.output
  File "apache_beam/runners/worker/operations.py", line 358, in apache_beam.runners.worker.operations.Operation.output
  File "apache_beam/runners/worker/operations.py", line 220, in apache_beam.runners.worker.operations.SingletonConsumerSet.receive
  File "apache_beam/runners/worker/operations.py", line 717, in apache_beam.runners.worker.operations.DoOperation.process
  File "apache_beam/runners/worker/operations.py", line 718, in apache_beam.runners.worker.operations.DoOperation.process
  File "apache_beam/runners/common.py", line 1235, in apache_beam.runners.common.DoFnRunner.process
  File "apache_beam/runners/common.py", line 1300, in apache_beam.runners.common.DoFnRunner._reraise_augmented
  File "apache_beam/runners/common.py", line 1233, in apache_beam.runners.common.DoFnRunner.process
  File "apache_beam/runners/common.py", line 581, in apache_beam.runners.common.SimpleInvoker.invoke_process
  File "apache_beam/runners/common.py", line 1395, in apache_beam.runners.common._OutputProcessor.process_outputs
  File "apache_beam/runners/worker/operations.py", line 220, in apache_beam.runners.worker.operations.SingletonConsumerSet.receive
  File "apache_beam/runners/worker/operations.py", line 717, in apache_beam.runners.worker.operations.DoOperation.process
  File "apache_beam/runners/worker/operations.py", line 718, in apache_beam.runners.worker.operations.DoOperation.process
  File "apache_beam/runners/common.py", line 1235, in apache_beam.runners.common.DoFnRunner.process
  File "apache_beam/runners/common.py", line 1300, in apache_beam.runners.common.DoFnRunner._reraise_augmented
  File "apache_beam/runners/common.py", line 1233, in apache_beam.runners.common.DoFnRunner.process
  File "apache_beam/runners/common.py", line 581, in apache_beam.runners.common.SimpleInvoker.invoke_process
  File "apache_beam/runners/common.py", line 1395, in apache_beam.runners.common._OutputProcessor.process_outputs
  File "apache_beam/runners/worker/operations.py", line 220, in apache_beam.runners.worker.operations.SingletonConsumerSet.receive
  File "apache_beam/runners/worker/operations.py", line 717, in apache_beam.runners.worker.operations.DoOperation.process
  File "apache_beam/runners/worker/operations.py", line 718, in apache_beam.runners.worker.operations.DoOperation.process
  File "apache_beam/runners/common.py", line 1235, in apache_beam.runners.common.DoFnRunner.process
  File "apache_beam/runners/common.py", line 1315, in apache_beam.runners.common.DoFnRunner._reraise_augmented
  File "/opt/conda/lib/python3.7/site-packages/future/utils/__init__.py", line 446, in raise_with_traceback
    raise exc.with_traceback(traceback)
  File "apache_beam/runners/common.py", line 1233, in apache_beam.runners.common.DoFnRunner.process
  File "apache_beam/runners/common.py", line 581, in apache_beam.runners.common.SimpleInvoker.invoke_process
  File "apache_beam/runners/common.py", line 1368, in apache_beam.runners.common._OutputProcessor.process_outputs
  File "/scratch/user_name/modules/datasets_modules/datasets/wikipedia/2fe8db1405aef67dff9fcc51e133e1f9c5b0106f9d9e9638188176d278fd5ff1/wikipedia.py", line 492, in _clean_content
    text = _parse_and_clean_wikicode(raw_content, parser=mwparserfromhell)
  File "/scratch/user_name/modules/datasets_modules/datasets/wikipedia/2fe8db1405aef67dff9fcc51e133e1f9c5b0106f9d9e9638188176d278fd5ff1/wikipedia.py", line 548, in _parse_and_clean_wikicode
    section_text.append(section.strip_code().strip())
  File "/opt/conda/lib/python3.7/site-packages/mwparserfromhell/wikicode.py", line 639, in strip_code
    stripped = node.__strip__(**kwargs)
  File "/opt/conda/lib/python3.7/site-packages/mwparserfromhell/nodes/html_entity.py", line 60, in __strip__
    return self.normalize()
  File "/opt/conda/lib/python3.7/site-packages/mwparserfromhell/nodes/html_entity.py", line 150, in normalize
    return chr(htmlentities.name2codepoint[self.value])
KeyError: "000nbsp [while running 'train/Clean content']"
```