connectd: also fail without a scary backtrace when listen fails.

For example, if you do:

```
./lightningd/lightningd --network=regtest --experimental-websocket-port=19846
```

Then you're trying to reuse the normal port as the websocket port, but this
only fails at *listen* time, when we activate connectd.  Catch this too.

Fixes incorrect fatal() message, too.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

connectd/connectd.c

@@ -1657,6 +1657,7 @@ static struct io_plan *(*get_in_cb(enum is_websocket is_websocket))(struct io_co
 static void connect_activate(struct daemon *daemon, const u8 *msg)
 {
 	bool do_listen;
+	char *errmsg = NULL;
 
 	if (!fromwire_connectd_activate(msg, &do_listen))
 		master_badmsg(WIRE_CONNECTD_ACTIVATE, msg);
@@ -1667,12 +1668,13 @@ static void connect_activate(struct daemon *daemon, const u8 *msg)
 			if (listen(daemon->listen_fds[i]->fd, 64) != 0) {
 				if (daemon->listen_fds[i]->mayfail)
 					continue;
-				status_failed(STATUS_FAIL_INTERNAL_ERROR,
-					      "Failed to listen on socket %s: %s",
-					      type_to_string(tmpctx,
-							     struct wireaddr_internal,
-							     &daemon->listen_fds[i]->wi),
-					      strerror(errno));
+				errmsg = tal_fmt(tmpctx,
+						 "Failed to listen on socket %s: %s",
+						 type_to_string(tmpctx,
+								struct wireaddr_internal,
+								&daemon->listen_fds[i]->wi),
+						 strerror(errno));
+				break;
 			}
 			notleak(io_new_listener(daemon,
 						daemon->listen_fds[i]->fd,
@@ -1687,7 +1689,7 @@ static void connect_activate(struct daemon *daemon, const u8 *msg)
 
 	/* OK, we're ready! */
 	daemon_conn_send(daemon->master,
-			 take(towire_connectd_activate_reply(NULL)));
+			 take(towire_connectd_activate_reply(NULL, errmsg)));
 }
 
 /* BOLT #10:

connectd/connectd_wire.csv

@@ -40,6 +40,7 @@ msgdata,connectd_activate,listen,bool,
 
 # Connectd->master, I am ready.
 msgtype,connectd_activate_reply,2125
+msgdata,connectd_activate_reply,failmsg,?wirestring,
 
 # connectd->master: disconnect this peer please (due to reconnect).
 msgtype,connectd_reconnected,2112

lightningd/connect_control.c

@@ -471,7 +471,7 @@ static void connect_init_done(struct subd *connectd,
 					  &ld->binding,
 					  &ld->announcable,
 					  &errmsg))
-		fatal("Bad connectd_activate_reply: %s",
+		fatal("Bad connectd_init_reply: %s",
 		      tal_hex(reply, reply));
 
 	/* connectd can fail in *informative* ways: don't use fatal() here and
@@ -549,10 +549,22 @@ int connectd_init(struct lightningd *ld)
 }
 
 static void connect_activate_done(struct subd *connectd,
-				  const u8 *reply UNUSED,
+				  const u8 *reply,
 				  const int *fds UNUSED,
 				  void *unused UNUSED)
 {
+	char *errmsg;
+	if (!fromwire_connectd_activate_reply(reply, reply, &errmsg))
+		fatal("Bad connectd_activate_reply: %s",
+		      tal_hex(reply, reply));
+
+	/* connectd can fail in *informative* ways: don't use fatal() here and
+	 * confuse things with a backtrace! */
+	if (errmsg) {
+		log_broken(connectd->log, "%s", errmsg);
+		exit(1);
+	}
+
 	/* Break out of loop, so we can begin */
 	io_break(connectd);
 }