Enable Subscription Resolver to return websocket error message

[zhixinwen]

Solve issue #1118

Currently there is no way of returning an error to client, after subscription resolver returns channel.
This is inconvenient because some clients have build-in retry on error message, and that is not going to work with gqlgen servers.

Ideally the subscriotion resolver should return an error channel as well, but this is going to be a breaking change.
Instead we can propagate error with context similar to graphql.AddError. This way the change would have no impact to existing users, and users can choose to opt into this behavior.

Tested on my local with resolver code:

func (r *SubscriptionResolver) Method(ctx context.Context, input models.Input) (<-chan *models.Response, error) {
	outputChan := make(chan *models.Response)

	streamClient, err := r.StreamClient.GetStream(ctx,toProto(input))
	if err != nil {
		return nil, mperror.Wrap(err, "fail to make grpc calls")
	}

	go func() {
		defer close(outputChan)
		for {
			outputProto, err :=  r.StreamClient.Recv()
			if err == io.EOF {
				return
			}

			if err != nil {
				transport.AddSubscriptionError(ctx, gqlerror.Errorf(err.Error()))
				return
			}

			outputModel := toModel(outputProto)

			select {
			case <-ctx.Done():
				return
			case outputChan <- outputModel:
				continue
			}
		}
	}()

	return outputChan, nil
}

![Screen Shot 2023-01-12 at 11 35 45 AM](https://user-images.githubusercontent.com/10200233/212168867-1024138f-a58a-4a56-8bd2-66ca2a02a13d.png)

According to the [spec](https://github.com/enisdenjo/graphql-ws/blob/master/PROTOCOL.md#error), there should be no more message after error message, and this change adheres to the spec



I have:
 - [ ] Added tests covering the bug / feature (see [testing](https://github.com/99designs/gqlgen/blob/master/TESTING.md))
 - [ ] Updated any relevant documentation (see [docs](https://github.com/99designs/gqlgen/tree/master/docs/content))

[coveralls]

Coverage: 75.542% (-0.09%) from 75.636% when pulling f1648cd on zhixinwen:zhixin/implement-subscription-error into 43c9a1d on 99designs:master.