[AMF] Add NGAP ErrorIndication [#910]

1. UE sends PDU session establishment request to the AMF.
2. AMF initiates Release Due to Duplicate Session ID.
3. SMF cannot find the session by SM-Context-Ref.

For the above condition, AMF sends NGAP ErrorIndication to the UE.

src/amf/nsmf-handler.c

@@ -602,10 +602,8 @@ int amf_nsmf_pdusession_handle_update_sm_context(
 
         ogs_error("[%d:%d] HTTP response error [%d]",
                 sess->psi, sess->pti, recvmsg->res_status);
-#if 0 /* Is it needed */
         ngap_send_error_indication2(amf_ue,
                 NGAP_Cause_PR_protocol, NGAP_CauseProtocol_semantic_error);
-#endif
 
         return OGS_ERROR;
     }