QBluetoothSocket::connectToService should report UnsupportedProtocolError early

It's possible to create a QBluetoothSocket with 'UnknownProtocol' type and then continue with connectToService. On OS X (for example) this results in UnknownError instead of documented/expected UknownProtocolError. Also, it's possible to call overloaded connectToService and this will result in undesired device discovery (undesired, since we'll fail to connect after all).