Missing torrent files crashes the torrent feed

[hubx]

If a torrent file is missing, the whole torrent cannot be created:

Traceback:
File "/opt/owntube-venv/local/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
  111.                         response = callback(request, *callback_args, **callback_kwargs)
File "/opt/owntube-venv/local/lib/python2.7/site-packages/django/contrib/syndication/views.py" in __call__
  38.         feedgen = self.get_feed(obj, request)
File "/opt/owntube-venv/local/lib/python2.7/site-packages/django/contrib/syndication/views.py" in get_feed
  157.                     length = smart_unicode(self.__get_dynamic_attr('item_enclosure_length', item)),
File "/opt/owntube-venv/local/lib/python2.7/site-packages/django/contrib/syndication/views.py" in __get_dynamic_attr
  71.                 return attr(obj)
File "./videoportal/feeds.py" in item_enclosure_length
  129.      return os.path.getsize(settings.BITTORRENT_FILES_DIR + item.slug + '.torrent')
File "/opt/owntube-venv/lib/python2.7/genericpath.py" in getsize
  49.     return os.stat(filename).st_size

Exception Type: OSError at /feeds/latest/torrent/
Exception Value: [Errno 2] No such file or directory: '/opt/OwnTube/media/torrents/vosi-20130528.torrent'

Check if file exists before asking for its size and creating an entry for it.