Download image script aborts as soon as an image cannot be downloaded

[yayayayaka]

When an image cannot be downloaded (for example when the hosting server has gone extinct) ,the script aborts with an AssertionError without downloading the rest of the images.

Stack Trace:

$ python download_images.py --no-thumbnails
Skipping 872 already-downloaded images
Downloading 73 new images...
Traceback (most recent call last):
  File "download_images.py", line 63, in <module>
    download_images()
  File "/home/naka/.local/share/virtualenvs/matrix-archive-j2607j-8/lib/python3.6/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/home/naka/.local/share/virtualenvs/matrix-archive-j2607j-8/lib/python3.6/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/home/naka/.local/share/virtualenvs/matrix-archive-j2607j-8/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/naka/.local/share/virtualenvs/matrix-archive-j2607j-8/lib/python3.6/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "download_images.py", line 59, in download_images
    run_downloads(new_messages, download_dir, prefer_thumbnails=thumbnails)
  File "download_images.py", line 23, in run_downloads
    assert res.status_code == 200
AssertionError

Rather than asserting a 200 response, I'd propose catching the requests.exceptions.RequestException and continue downloading the next images.