Add network checks
Based on checks made for #5301 (closed) we can improve the checks merged in https://github.com/leapcode/bitmask_client/pull/557. So we can improve the user messages in case of connectivity problems.
(from redmine: created on 2014-03-13, relates #4023 (closed), relates #4585 (closed), relates #4823 (closed), relates #4765 (closed), relates #1337 (closed), relates #5725, duplicates #6059 (closed))