code: func (s *GlobalStore) Get(addr common.Address, key []byte) (data []byte, err error) {
  	err = s.client.Call(&data, "mockStore_get", addr, key)
  	if err != nil && err.Error() == "not found" {
  		// pass the mock package value of error instead an rpc error
  		return data, mock.ErrNotFound
  	}
  	return data, err
  }
docs: // Get calls a Get method to RPC server.