GUI: handle aborted request (can't contact server)

- When request to server is aborted by browser (can't contact server), then
 status code of response is 0. Request must be handled in onResponseRecieved().

- Fixed display of connection lost widget in main GUI. Display it only
  when request is aborted by browser.
Merge pull request #565 from zlamalp/gui

GUI: handle aborted request (can't contact server)