QBluetoothServiceDiscoveryAgent returns errors for when it should not on Symbian

This is an new addition required to fix the QBluetoothServiceDiscoveryAgent unit test.

QBluetoothServiceDiscoveryAgent returns errors for any error that occurs during service discovery. This can be misleading. When doing a scan and discovering services on all contactable devices they may not respond to SDP, or maybe travel out of range before SDP can be performed. Errors in this case can be safely ignored since they don't affect the user.

On the other hand, if QBluetoothServiceDiscoveryAgent(QBluetoothAddress) is used, service discovery is done on a single device and errors are important and should be returned and emitted.

QBluetoothServiceDiscoveryAgentPrivate::singleDevice provides a bool to be able to provide this error handling, it is currently not being used.