* add function to check whether cu-up is connected
* add functions to send the relevant response messages with the failed list
e.g. to fill PDU Session Resource Failed to Setup items
* set return value of trigger_bearer_setup to bool and handle the
failure by sending setup response messages with list of PDU sessions failed to setup
i.e. send failed list of items when no CU-UP is associated to the CU-CP
note: it considers all PDU Sessions as failed to setup in the considered failure scenarios
* e.g. send a list of failed PDU session resources in the response message to reject
PDU Session Resource Setup in rrc_gNB_process_NGAP_PDUSESSION_SETUP_REQ
* the Initial Context Setup has a Failure message, but for the considered failure scenario
when no CU-UP is available the current design is just sending a list of failed to setup
items in the Response message
* transfer PDU Session Resource/Initial Context Setup Request NAS PDU to UE
closes#878
Guido Casati
a1abf90d5b