Survive a stopped dockerd; report unexpected failures

- no docker0 yet (dockerd is socket-activated and starts on first use): fall back to the
  default gateway instead of exiting silently under pipefail
- an ERR trap in the main process prints file, line and command of an unhandled failure
- egress layers list returns success when the last layer is absent
- drop the stale OCSB_EGRESS_BASE from the settings list (broke `config`)

lib/egress.sh

@@ -26,6 +26,7 @@ ocsb_egress_layers() {
     "${OCSB_PROJECT_DIR}/.opencode-sandbox/local/egress.conf"; do
     [[ -f ${f} ]] && echo "${f}"
   done
+  return 0
 }
 
 # Prints "directive<TAB>arg" for every argument of every rule line