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 a eRAB but
the co-located UPF is not available at the time.

This patch all the generic infrastructure to be able to reply (errors)
and implements the above mentioned specific case.

Change-Id: I242e84fb09b00f4794b6e1aa770f348a0e60aea4