Improve NavigationServer.free() error msg when RID not found

Improves the error msg when NavigationServer.free() does not find the RID e.g. because it was already deleted or did never exist in the first place.

modules/navigation/godot_navigation_server.cpp

@@ -757,7 +757,7 @@ COMMAND_1(free, RID, p_object) {
 		agent_owner.free(p_object);
 
 	} else {
-		ERR_FAIL_COND("Invalid ID.");
+		ERR_FAIL_COND("Attempted to free a NavigationServer RID that did not exist (or was already freed).");
 	}
 }