ignore http error

[smdmts]

If slack's emoji alias was broken, the following error appears. So requiring error handling.

Traceback (most recent call last):
  File "get_emojis.py", line 54, in <module>
    main(args)
  File "get_emojis.py", line 41, in main
    download_image(image_url, image_path)
  File "get_emojis.py", line 15, in download_image
    r = requests.get(url, stream=True)
  File "/Users/b06193/.pyenv/versions/2.7.11/lib/python2.7/site-packages/requests/api.py", line 71, in get
    return request('get', url, params=params, **kwargs)
  File "/Users/b06193/.pyenv/versions/2.7.11/lib/python2.7/site-packages/requests/api.py", line 57, in request
    return session.request(method=method, url=url, **kwargs)
  File "/Users/b06193/.pyenv/versions/2.7.11/lib/python2.7/site-packages/requests/sessions.py", line 475, in request
    resp = self.send(prep, **send_kwargs)
  File "/Users/b06193/.pyenv/versions/2.7.11/lib/python2.7/site-packages/requests/sessions.py", line 579, in send
    adapter = self.get_adapter(url=request.url)
  File "/Users/b06193/.pyenv/versions/2.7.11/lib/python2.7/site-packages/requests/sessions.py", line 653, in get_adapter
    raise InvalidSchema("No connection adapters were found for '%s'" % url)
requests.exceptions.InvalidSchema: No connection adapters were found for 'alias:ace'