Issue a 404 instead of 400 for unknown short code requests

[mshmsh5000]

When handling a GET <key>, if the key isn't in the database, right now we return a 400 Bad Request:

    return jsonify({'error': 'url not found'}, 400)

This only makes sense in terms of the internal request. The requester would expect to get a 404 here.

[mshmsh5000]

h/t @rapala61

[mshmsh5000]

Closed with #19