get_thumbnail_url crashes when image is missing

Sentry Issue: CONCREXIT-Q

FileNotFoundError: [Errno 2] No such file or directory: '/concrexit/media/public/avatars/bestuursfoto.jpg'
(26 additional frame(s) were not displayed)
...
  File "django/template/library.py", line 214, in render
    _dict = self.func(*resolved_args, **resolved_kwargs)
  File "activemembers/templatetags/activemembers_cards.py", line 47, in membergroup_member_card
    ribbon=ribbon
  File "members/templatetags/member_card.py", line 22, in member_card
    settings.THUMBNAIL_SIZES['medium'])
  File "utils/media/services.py", line 90, in get_thumbnail_url
    os.path.getmtime(full_original_path)
  File "genericpath.py", line 55, in getmtime
    return os.stat(filename).st_mtime