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.