Script crashes in case of a single airport download issue.

[k-popov]

All download fails if a single airport can't be downloaded:

INFO:root:[29910/34766] 18TE is being processed
INFO:root:Getting APT data for 18TE
INFO:root:Getting scenery for 8344
INFO:requests.packages.urllib3.connectionpool:Starting new HTTP connection (1): gateway.x-plane.com
Traceback (most recent call last):
  File "apt_fetcher.py", line 208, in <module>
    main()
  File "apt_fetcher.py", line 191, in main
    apt_data = get_airport_apt(scenery)
  File "apt_fetcher.py", line 41, in get_airport_apt
    scenery_json = get_json_from_api(scenery_request)
  File "apt_fetcher.py", line 17, in get_json_from_api
    reply = requests.get(api_request)
  File "/home/hub/fg_tools/local/lib/python2.7/site-packages/requests/api.py", line 68, in get
    return request('get', url, **kwargs)
  File "/home/hub/fg_tools/local/lib/python2.7/site-packages/requests/api.py", line 50, in request
    response = session.request(method=method, url=url, **kwargs)
  File "/home/hub/fg_tools/local/lib/python2.7/site-packages/requests/sessions.py", line 464, in request
    resp = self.send(prep, **send_kwargs)
  File "/home/hub/fg_tools/local/lib/python2.7/site-packages/requests/sessions.py", line 576, in send
    r = adapter.send(request, **kwargs)
  File "/home/hub/fg_tools/local/lib/python2.7/site-packages/requests/adapters.py", line 415, in send
    raise ConnectionError(err, request=request)
requests.exceptions.ConnectionError: ('Connection aborted.', error(110, 'Connection timed out'))

We need to have retries but if download of this exact file fails after all retries, skip it leaving some kind of special log message.

[k-popov]

Implemented in 42b017c