Issue more precise error when disconnecting a nonexistent connection

Checks whether the signal exists when issuing an error message when
disconnecting a nonexistent connection. Also prints the callable name.

(cherry picked from commit 6c026a6814eb15c2832f9cfd30ee8ef8d0a236ab)

core/object.cpp

@@ -1563,7 +1563,13 @@ void Object::_disconnect(const StringName &p_signal, Object *p_to_object, const
 
 	ERR_FAIL_NULL(p_to_object);
 	Signal *s = signal_map.getptr(p_signal);
-	ERR_FAIL_COND_MSG(!s, vformat("Nonexistent signal '%s' in %s.", p_signal, to_string()));
+	if (!s) {
+		bool signal_is_valid = ClassDB::has_signal(get_class_name(), p_signal) ||
+							   (!script.is_null() && Ref<Script>(script)->has_script_signal(p_signal));
+		ERR_FAIL_COND_MSG(signal_is_valid, vformat("Attempt to disconnect a nonexistent connection to signal '%s' in %s, with target '%s' in %s.",
+												   p_signal, to_string(), p_to_method, p_to_object->to_string()));
+	}
+	ERR_FAIL_COND_MSG(!s, vformat("Disconnecting nonexistent signal '%s' in %s.", p_signal, to_string()));
 
 	Signal::Target target(p_to_object->get_instance_id(), p_to_method);