SecureSocketClient: Show general connection errors

So far the example didn't show any errors when the connection to the
server couldn't be established (e.g. because the host/port number was
wrong, but also because the SSL handshake already failed).

The fix shows a modal error dialog in this case.

Change-Id: Ia517bf79c5fb2ccdb1edabded28f86759f3e6dcf
Reviewed-by: Richard J. Moore <rich@kde.org>