Fix error message when using is_connected with a script signal

core/object.cpp

@@ -1405,6 +1405,10 @@ bool Object::is_connected(const StringName& p_signal, Object *p_to_object, const
 		bool signal_is_valid = ObjectTypeDB::has_signal(get_type_name(),p_signal);
 		if (signal_is_valid)
 			return false;
+
+		if (!script.is_null() && Ref<Script>(script)->has_script_signal(p_signal))
+			return false;
+
 		ERR_EXPLAIN("Nonexistent signal: "+p_signal);
 		ERR_FAIL_COND_V(!s,false);
 	}