Changes

s1ap_proxy: Support replying errors

Sometimes it is needed not only to forward, but to transmit messages
initiated at the S1GW.  This is the case, for instance, if the MME
wants to initiate an E-RAB establishment, but the co-located UPF is
not available at the time or responds with an error.

This patch adds the generic infrastructure to be able to reply
(errors) and implements the above mentioned specific case for
the S1AP E-RAB SETUP REQUEST procedure.

Change-Id: I242e84fb09b00f4794b6e1aa770f348a0e60aea4