[PATCH] Return error if lockdown fails starting a service

The codepath of "iphone_lckd_start_service" currently fails for the case when the requested service could not be started and a "Result:Failure" is returned from the device. It returns the "ret" variable of the last iphone_mux_recv() call which will still be IPHONE_E_SUCCESS.

This also leads to issues with the Python API as the code below would never trigger if the service fails to start:

self._mobile_sync = self._lckd.get_mobile_sync_client()
if not self._mobile_sync:
    # never reached
    print "Mobilesync session couldn't be established."
    sys.exit(-1)

Attached patch fixes this case and also introduces a new error code for this case.