code does not handle "S1AP Error Indication" from eNB

[spencersevilla]

Seeing a warning message in s1ap-sm.c:121 for (choice:1 proc:15) because we have not implemented.
(choice 1 proc 15) corresponds to receiving a S1AP Error Indication from the eNB as part of the Initiating Message.

We should check the 3GPP to see how to handle. I assume the correct response is to drop silently (since we have no procedure to abort).