We did not give a cause value, and therefore NGAP's "PDU sessions
failed" encoding failed. Give a cause value in all cases, so that this
cannot happen anymore.

To prevent against this, put an AssertFatal() into the encoder that
stops if an internally sent message does not have a correct cause value
(we can at least select a default).

Print an info message for all failed PDU sessions.

In rrc_gNB_send_NGAP_INITIAL_CONTEXT_SETUP_RESP(): if a PDU session is
already established, don't count it as failed.