Macro rpc_error_obj 

macro_rules! rpc_error_obj {
    ($code:expr, $message:expr, $data:expr) => { ... };
    ($code:expr, $message:expr) => { ... };
}

Create a JSON-RPC error object (not a response)

Usage:

// Error with code and message
rpc_error_obj!(-32602, "Invalid parameters")

// Error with code, message and data
rpc_error_obj!(-32602, "Invalid parameters", {"expected": "array"})