Import Documents: crash on bad URL

Describe the bug

--------------------------- RuntimeError Exception ----------------------------
Traceback (most recent call last):
  File "/Users/ajda/orange/orange3-text/orangecontrib/text/import_documents.py", line 244, in _read_files
    return await asyncio.gather(*req)
  File "/Users/ajda/orange/orange3-text/orangecontrib/text/import_documents.py", line 256, in _read_file
    return UrlProxyReader._parse_response(response)
  File "/Users/ajda/orange/orange3-text/orangecontrib/text/import_documents.py", line 273, in _parse_response
    text_data.ext,
AttributeError: 'NoneType' object has no attribute 'ext'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/ajda/orange/orange3-text/orangecontrib/text/widgets/owimportdocuments.py", line 611, in __onRunFinished
    corpus, errors, lemmas, pos, ner, is_conllu = task.future.result()
  File "/Users/ajda/.pyenv-x86/versions/3.9.10/lib/python3.9/concurrent/futures/_base.py", line 439, in result
    return self.__get_result()
  File "/Users/ajda/.pyenv-x86/versions/3.9.10/lib/python3.9/concurrent/futures/_base.py", line 391, in __get_result
    raise self._exception
  File "/Users/ajda/orange/orange3/Orange/widgets/utils/concurrent.py", line 175, in run
    result = func(*args, **kwargs)
  File "/Users/ajda/orange/orange3-text/orangecontrib/text/widgets/owimportdocuments.py", line 589, in run_text_scan_task_interupt
    return task.run()
  File "/Users/ajda/orange/orange3-text/orangecontrib/text/import_documents.py", line 399, in run
    self._text_data, errors_text, tokens, pos, ner, conllu = self._read_text_data(
  File "/Users/ajda/orange/orange3-text/orangecontrib/text/import_documents.py", line 443, in _read_text_data
    results = UrlProxyReader().read_files(paths, callback)
  File "/Users/ajda/orange/orange3-text/orangecontrib/text/import_documents.py", line 238, in read_files
    return asyncio.run(UrlProxyReader._read_files(urls, callback))
  File "/Users/ajda/.pyenv-x86/versions/3.9.10/lib/python3.9/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/Users/ajda/.pyenv-x86/versions/3.9.10/lib/python3.9/asyncio/base_events.py", line 642, in run_until_complete
    return future.result()
  File "/Users/ajda/orange/orange3-text/orangecontrib/text/import_documents.py", line 244, in _read_files
    return await asyncio.gather(*req)
  File "/Users/ajda/.pyenv-x86/versions/3.9.10/envs/py3.9/lib/python3.9/site-packages/httpx/_client.py", line 1975, in __aexit__
    await self._transport.__aexit__(exc_type, exc_value, traceback)
  File "/Users/ajda/.pyenv-x86/versions/3.9.10/envs/py3.9/lib/python3.9/site-packages/httpx/_transports/default.py", line 332, in __aexit__
    await self._pool.__aexit__(exc_type, exc_value, traceback)
  File "/Users/ajda/.pyenv-x86/versions/3.9.10/envs/py3.9/lib/python3.9/site-packages/httpcore/_async/connection_pool.py", line 326, in __aexit__
    await self.aclose()
  File "/Users/ajda/.pyenv-x86/versions/3.9.10/envs/py3.9/lib/python3.9/site-packages/httpcore/_async/connection_pool.py", line 312, in aclose
    raise RuntimeError(
RuntimeError: The connection pool was closed while 1 HTTP requests/responses were still in-flight.
-------------------------------------------------------------------------------

To Reproduce
Steps to reproduce the behavior:

1. Import Documents
2. Pass a bad URL, I entered http://file.biolab.si/text-semantics/, which is a folder of many subfolders.

Expected behavior
Error of some sort is reported.

Orange version:
3.32.dev

Text add-on version:
1.7.dev

Start in orangecontrib/text/import_documents.py at UrlProxyReader._read_files, _read_file, and _parse_response, then trace how errors reach __onRunFinished in orangecontrib/text/widgets/owimportdocuments.py. Reproduce the issue through Import Documents with the bad folder URL, and verify that an error is reported without either AttributeError or the connection-pool RuntimeError.