zenoh: Default to 127.0.0.1 when using sitl/posix

Also improve error message when connection failed

src/modules/zenoh/Kconfig

@@ -16,6 +16,11 @@ if MODULES_ZENOH
             2: INFO + ERROR
             3: DEBUG + INFO + ERROR
 
+    config ZENOH_DEFAULT_LOCATOR
+        string "Zenoh default mode"
+        default "tcp/127.0.0.1:7447" if PLATFORM_POSIX
+        default "" if !PLATFORM_POSIX
+
     # Choose exactly one item
     choice ZENOH_PUBSUB_SELECTION
             prompt "Publishers/Subscribers selection"

src/modules/zenoh/zenoh.cpp

@@ -126,7 +126,10 @@ void ZENOH::run()
 	ret = z_open(&s, z_move(config), NULL);
 
 	if (ret < 0) {
-		if (ret == _Z_ERR_SCOUT_NO_RESULTS) {
+		if (ret == _Z_ERR_TRANSPORT_OPEN_FAILED) {
+			PX4_ERR("Unable to open session, make sure zenohd is running on %s", locator);
+
+		} else if (ret == _Z_ERR_SCOUT_NO_RESULTS) {
 			PX4_ERR("Unable to open session, scout no results");
 
 		} else {

src/modules/zenoh/zenoh_config.cpp

@@ -50,7 +50,7 @@
 #include <uORB/topics/uORBTopics.hpp>
 
 
-const char *default_net_config = Z_CONFIG_MODE_DEFAULT;
+const char *default_net_config = Z_CONFIG_MODE_DEFAULT ";" CONFIG_ZENOH_DEFAULT_LOCATOR;
 const char *default_pub_config = "";
 const char *default_sub_config = ""; //TODO maybe use YAML