Support Error Extensions

[nicolaiunrein]

Hey,
first of all, I really enjoy using your library so far. I think it is already very comprehensive. One thing I am missing though is returning extensions for errors as described here in the spec. Something like:

  "errors": [
    {
      "message": "Name for character with ID 1002 could not be fetched.",
      "locations": [ { "line": 6, "column": 7 } ],
      "path": [ "hero", "heroFriends", 1, "name" ],
      "extensions": {
        "code": "CAN_NOT_FETCH_BY_ID",
        "timestamp": "Fri Feb 9 14:33:09 UTC 2018"
      }
    }
  ]
}

In terms of api, I think it would be nice to have a trait with a single method with_extensions which you could implement on your errors. Something like:

// somewhere in async-graphql
pub trait ErrorExtensions {
    fn with_extensions(&self) -> Option<serde_json::Value> {
        None
    }
}

impl<T: Into<Error>> ErrorExtensions for T {}

// somewhere in users code...
impl ErrorExtensions for MyError {
  fn with_extensions(&self) -> Option<serde_json::Value> {

    let extensions = serde_json::json!({
       "code": self.code.to_string(),
       "timestamp": get_current_timestamp()
    });

    Some(extensions)
  }
}

I am happy to contribute, so if you want any help just let me know.

[sunli829]

Thank you for your advice, that's a great idea, and I really need your help. These days I want to optimize the code in the subscription to make it easier to use, so our work doesn't conflict.If you have time, you can help implement `ErrorExtensions'.😁
Thanks again!

[nicolaiunrein]

I thought a little about it and started implementing a first draft. Will be ready for review tomorrow or on monday I think.

[sunli829]

Is a wrapper similar to PositionError?

[nicolaiunrein]

Is a wrapper similar to PositionError?

Yes exactly. See PR for details.

[sunli829]

I reimplemented the error type in branch error-type, the error extension is still not very good, give some advice?

[nicolaiunrein]

I looked at the error-type and made some adjustments. Please have a look and also at the example uses cases. It turns out to be quite tricky to implement some sort of default implementation but still let the user implement custom behaviour. Feel free to undo the commit if you don't like it.

[nicolaiunrein]

I also replaced the trait bound std::err::Error with std::fmt::Display because I think Error is an unnecessary restriction.

[sunli829]

Thank you, It's merged to master.😁