Allow resolving exceptions from a subscription

[Munoon]

As a user, I want to be able to create a custom exception mapper in GraphQLError inside subscriptions. It is currently impossible to modify the error response, if you throw an exception inside the method, annotated with @SubscriptionMapping or inside publisher, that returning method with @SubscriptionMapping annotation.

Here is an example use case: when the user creates a subscription, I want to verify if he can subscribe. If not, I want to return an error, with details inside the extensions field.

Currently, each exception is mapping to the following error response:

{
    "message": "Exception error",
    "locations": [],
    "extensions": { "classification": "DataFetchingException" }
}

I also create a small application, that represents this problem: example.zip

[rstoyanchev]

Thanks for raising this.

Exceptions from a subscription are separate from the DataFetcherExceptionHandler mechanism. The seem to flow directly into the transport layer which is subscribed to the Publisher. We'll need to expose a hook through WebSocketGraphQlInterceptor for this, probably taking an exception and returning List<GraphQLError>.

[rstoyanchev]

I've now also seen PR #398. Let's continue the discussion there until we decide on the implementation details.

[rstoyanchev]

Superseded by #398.