[DS] Response fail instead of not response at all

when not able to get subscriber number (return null),
HFP shall response to remote devie with Error instead of
not responding at all.

Change-Id: I4dd90071b72d33de14c69f98a1360a4f53a8bb3f

Conflicts:
	src/com/android/bluetooth/hfp/HeadsetStateMachine.java