Improve jsonrpc error reporting

(cherry picked from commit dac8b7b6f4fec0136a3dc95afae0aa8aa8ce9fd9)

modules/jsonrpc/jsonrpc.cpp

@@ -120,7 +120,7 @@ Variant JSONRPC::process_action(const Variant &p_action, bool p_process_arr_elem
 		}
 
 		if (object == nullptr || !object->has_method(method)) {
-			ret = make_response_error(JSONRPC::METHOD_NOT_FOUND, "Method not found", id);
+			ret = make_response_error(JSONRPC::METHOD_NOT_FOUND, "Method not found: " + method, id);
 		} else {
 			Variant call_ret = object->callv(method, args);
 			if (id.get_type() != Variant::NIL) {