Change error returns in subsession_register_event_callback and subsession_unregister_...

Change error returns in subsession_register_event_callback and subsession_unregister_event_callbacks

subsession_register_event will return SUBSESSION_ERROR_ALREADY_EXISTS
if event on the same subsession_uid was previously registered

subsession_unregister_event will return SUBSESSION_ERROR_NOT_AVAILABLE
if event on the subsession wasn't or previously unregistered

to change callback and callbacks data
unregister and register event again is required

To update the calllback and callback's data the call of subsession_unregister_event_callback
and subsession_register_event_callback with new data is required.

Change-Id: I2ad90f7ac04e9e5f8417f5842db920e91c94511c