bug: convert.py crashes if downloading episode fails

[noman-land]

-- 06:59:20 -- Episode 296 -- Downloading at https://pscrb.fm/rss/p/pdst.fm/e/arttrk.com/p/ABMA5/audioboom.com/posts/7431775.mp3?modified=1657034312&sid=2399216&source=rss.
Traceback (most recent call last):
  File "/dev/nosuchthing/src/convert.py", line 7, in <module>
    fetch.download_audio(episode)
  File "/dev/nosuchthing/src/fetch.py", line 15, in download_audio
    urllib.request.urlretrieve(audio_url, audio_path)
  File "/.pyenv/versions/3.11.3/lib/python3.11/urllib/request.py", line 241, in urlretrieve
    with contextlib.closing(urlopen(url, data)) as fp:
                            ^^^^^^^^^^^^^^^^^^
  File "/.pyenv/versions/3.11.3/lib/python3.11/urllib/request.py", line 216, in urlopen
    return opener.open(url, data, timeout)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/.pyenv/versions/3.11.3/lib/python3.11/urllib/request.py", line 525, in open
    response = meth(req, response)
               ^^^^^^^^^^^^^^^^^^^
  File "/.pyenv/versions/3.11.3/lib/python3.11/urllib/request.py", line 634, in http_response
    response = self.parent.error(
               ^^^^^^^^^^^^^^^^^^
  File "/.pyenv/versions/3.11.3/lib/python3.11/urllib/request.py", line 557, in error
    result = self._call_chain(*args)
             ^^^^^^^^^^^^^^^^^^^^^^^
  File "/.pyenv/versions/3.11.3/lib/python3.11/urllib/request.py", line 496, in _call_chain
    result = func(*args)
             ^^^^^^^^^^^
  File "/.pyenv/versions/3.11.3/lib/python3.11/urllib/request.py", line 749, in http_error_302
    return self.parent.open(new, timeout=req.timeout)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/.pyenv/versions/3.11.3/lib/python3.11/urllib/request.py", line 525, in open
    response = meth(req, response)
               ^^^^^^^^^^^^^^^^^^^
  File "/.pyenv/versions/3.11.3/lib/python3.11/urllib/request.py", line 634, in http_response
    response = self.parent.error(
               ^^^^^^^^^^^^^^^^^^
  File "/.pyenv/versions/3.11.3/lib/python3.11/urllib/request.py", line 557, in error
    result = self._call_chain(*args)
             ^^^^^^^^^^^^^^^^^^^^^^^
  File "/.pyenv/versions/3.11.3/lib/python3.11/urllib/request.py", line 496, in _call_chain
    result = func(*args)
             ^^^^^^^^^^^
  File "/.pyenv/versions/3.11.3/lib/python3.11/urllib/request.py", line 749, in http_error_302
    return self.parent.open(new, timeout=req.timeout)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/.pyenv/versions/3.11.3/lib/python3.11/urllib/request.py", line 525, in open
    response = meth(req, response)
               ^^^^^^^^^^^^^^^^^^^
  File "/.pyenv/versions/3.11.3/lib/python3.11/urllib/request.py", line 634, in http_response
    response = self.parent.error(
               ^^^^^^^^^^^^^^^^^^
  File "/.pyenv/versions/3.11.3/lib/python3.11/urllib/request.py", line 563, in error
    return self._call_chain(*args)
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/.pyenv/versions/3.11.3/lib/python3.11/urllib/request.py", line 496, in _call_chain
    result = func(*args)
             ^^^^^^^^^^^
  File "/.pyenv/versions/3.11.3/lib/python3.11/urllib/request.py", line 643, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)
urllib.error.HTTPError: HTTP Error 503: Service Unavailable: Back-end server is at capacity