`UnicodeDecodeError` when reading csv file

Sentry Issue: OPENCODELISTS-TF

UnicodeDecodeError: 'utf-8' codec can't decode byte 0x83 in position 11: invalid start byte
decoding with 'utf-8-sig' codec failed
(8 additional frame(s) were not displayed)
...
  File "codelists/views/decorators.py", line 40, in wrapped_view
    return view_fn(request, codelist)
  File "codelists/views/decorators.py", line 139, in wrapped_view
    return view_fn(request, obj, *args, **kwargs)
  File "codelists/views/version_upload.py", line 19, in version_upload
    return handle_post(request, codelist)
  File "codelists/views/version_upload.py", line 30, in handle_post
    if form.is_valid():
  File "codelists/forms.py", line 95, in clean_csv_data
    data = self.cleaned_data["csv_data"].read().decode("utf-8-sig")

Seems this could be caused by non-decodable data in the file. Ideally we would handle this case and provide a sensible message to user.

Start in codelists/forms.py at clean_csv_data, line 95, where uploaded CSV data is decoded as utf-8-sig. Reproduce the upload with a file containing non-decodable data and inspect the surrounding form validation; done means the user receives a sensible validation message instead of an unhandled UnicodeDecodeError.