code: function toRpcError(rpcCode, cause) {
    var err = toError(rpcCode.toLowerCase().replace(/_/g, ' '), cause);
    err.rpcCode = (cause && cause.rpcCode) ? cause.rpcCode : rpcCode;
    return err;
  }
docs: Mark an error.
  
  @param rpcCode {String} Code representing the failure.
  @param cause {Error} The cause of the error. It is available as `cause`
  field on the outer error.
  
  This is used to keep the argument of channels' `'error'` event errors.