snabb config listen: fix bug introduced in bfa68ba

src/program/config/listen/listen.lua

@@ -95,7 +95,14 @@ function run(args)
       common.print_and_exit(args)
    end
    local caller = rpc.prepare_caller('snabb-config-leader-v1')
-   local leader = common.open_socket_or_die(args.instance_id)
+   local leader = common.open_socket(args.instance_id)
+   if not leader then
+      io.stderr:write(
+         ("Could not connect to config leader socket on Snabb instance %q\n")
+            :format(args.instance_id)
+      )
+      main.exit(1)
+   end
    attach_listener(leader, caller, args.schema_name, args.revision_date)
 
    local handler = require('lib.fibers.file').new_poll_io_handler()