s1ap_proxy: fix build_erab_setup_response_failure() to report all E-RABs
The previous implementation only included the first E-RAB from the
registry in the failure response (a FIXME was left in place).
Fix it to extract all E-RAB IDs from the original E-RAB SETUP REQUEST
and include each of them in the failure list, as required by 3GPP.
Change-Id: I7933fceb0edcdfdc95ace35416297b11c83f0bc9
Related: osmo-ttcn3-hacks.git I8a5dc338d28013dc85e1ce4b3bdac92cb3b35304
