download: fail gracefully if no screens were shared during the session

[rohieb]

I got the following stack trace, which is solved by this PR:

Downloading https://bbb.stratum0.org/presentation/b3f9875e5e8180697cd3d3029085a7ef441d513d-1744649786905/deskshare/deskshare.webm...
Traceback (most recent call last):
  File "download.py", line 85, in <module>
    sys.exit(main(sys.argv))
             ^^^^^^^^^^^^^^
  File "download.py", line 81, in main
    d.download()
  File "download.py", line 73, in download
    self._get('deskshare/deskshare.webm')
  File "download.py", line 38, in _get
    resp = urllib.request.urlopen(req)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/urllib/request.py", line 216, in urlopen
    return opener.open(url, data, timeout)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/urllib/request.py", line 525, in open
    response = meth(req, response)
               ^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/urllib/request.py", line 634, in http_response
    response = self.parent.error(
               ^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/urllib/request.py", line 563, in error
    return self._call_chain(*args)
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/urllib/request.py", line 496, in _call_chain
    result = func(*args)
             ^^^^^^^^^^^
  File "/usr/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 404: Not Found

The presentation URL is public, you should be able to reproduce this issue.