Stop waiting in QTcpServerConnection if waitForBytesWritten fails

There is no point in waiting any further for the remaining bytes to be
written as it will never succeed. We might get luckier by creating a
local event loop and repeatedly calling processEvents(), but as that is
considerably worse style and because you shouldn't rely on the
connection to send on exit anyway we don't do that.

Task-number: QTBUG-42158, see also QTBUG-24451
Change-Id: I79ffd5f5a4a7c41ff8dc0c4f6f1ca7e091844c9d
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>