Need error handling or processing of non media posts.

[cmullins83]

Getting the following error occasionally:

     * This is a redgif link
       - Looking for submission.preview.reddit_video_preview.fallback_url
Traceback (most recent call last):
  File "/usr/lib/python3.8/runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.8/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/home/christopher/saveddit/saveddit/saveddit.py", line 65, in <module>
    main(args)
  File "/home/christopher/saveddit/saveddit/saveddit.py", line 31, in main
    downloader.download(args.o,
  File "/home/christopher/saveddit/saveddit/subreddit_downloader.py", line 141, in download
    self.download_gfycat_or_redgif(submission, files_dir)
  File "/home/christopher/saveddit/saveddit/subreddit_downloader.py", line 371, in download_gfycat_or_redgif
    if "reddit_video_preview" in submission.preview:
  File "/home/christopher/.local/lib/python3.8/site-packages/praw/models/reddit/base.py", line 35, in __getattr__
    return getattr(self, attribute)
  File "/home/christopher/.local/lib/python3.8/site-packages/praw/models/reddit/base.py", line 36, in __getattr__
    raise AttributeError(
AttributeError: 'Submission' object has no attribute 'preview'