Better Error handling

[Gers2017]

Return Error instead of String to the user

async fn send_to_node(&self, json: &str) -> Result<String, String> {
                                                             ^----
 /// ...
  let operation_result = serde_json::from_str(json);

  let operation: PlainOperation = match operation_result {
    Ok(op) => op,
    Err(err) => return Err(err.to_string()),
                                ^----
  };
}