MINOR: protocol: uniformize protocol errors

Some protocols fail with "error blah [ip:port]" and other fail with
"[ip:port] error blah". All this already appears in a "starting" or
"binding" context after a proxy name. Let's choose a more universal
approach like below where the ip:port remains at the end of the line
prefixed with "for".

  [WARNING]  (18632) : Binding [binderr.cfg:10] for proxy http: cannot bind receiver to device 'eth2' (No such device) for [0.0.0.0:1080]
  [WARNING]  (18632) : Starting [binderr.cfg:10] for proxy http: cannot set MSS to 12 for [0.0.0.0:1080]
diff --git a/src/proto_udp.c b/src/proto_udp.c
index f07170f..c5ecfac 100644
--- a/src/proto_udp.c
+++ b/src/proto_udp.c
@@ -148,7 +148,7 @@
 		char pn[INET6_ADDRSTRLEN];
 
 		addr_to_str(&listener->rx.addr, pn, sizeof(pn));
-		snprintf(errmsg, errlen, "%s [%s:%d]", msg, pn, get_host_port(&listener->rx.addr));
+		snprintf(errmsg, errlen, "%s for [%s:%d]", msg, pn, get_host_port(&listener->rx.addr));
 	}
 	return err;
 }