async-graphql: Error extensions

Hi @sunli829,

Thanks a lot for implementing this! https://github.com/async-graphql/async-graphql/issues/671

Maybe I don’t know how to handle this but basically I would like to achieve this:

• I want to add code to error.extenstions
• I also would like resolver to return MyError as I want to capture some types of errors (blanket using From) and do some special handling for them.

So I would like my resolver to look like this, but seems like the only way to add error.extenstions is to return async_graphql::Error instead? unless I create a separate method to return MyError and change my resolver to return async_graphql::Error?

#[Object]
impl TestMutation {
    async fn test(&self, ctx: &Context<'_>) -> Result<Test, MyError> {