Add error flag for use case when the remote peer closes the connection

[ChangeLog][QtBluetooth][QBluetoothSocket] Added
QBluetoothSocket::RemoteHostClosedError.

Change-Id: Ic9d27f9188f66be0fad309e55f2904eaed2cf61a
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>