Fix errors handling in GraphQL subscriptions

[50U10FCA7]

Synopsis

juniper_graphql_ws doesn't allow end-user to handle panics can happen during resolvers/streams execution.

Also, current implementation of #[graphql_subscription] maps errors incorrectly, which causes memory leak and completely omits errors in GraphQL subscriptions:

Omitted errors in GraphQL subscriptions

Each subscription resolves items of its stream, expecting the resolving can fail which is never happens because of the GraphQL specification. And because of this, no errors passed to GraphQL subscription leaving them inside the Executor.

Memory leak

juniper::Executor is shared across all the subscription's items (one executor per subscription). And its errors shared across all the types resolution. Executor::errors in current implementation only collects errors but never Drops them, which can lead to a memory leak in long-running subscription regularly emitting errors.

Solution

• Support panic handling in juniper_graphql_ws crate.
• Fix errors mapping in #[graphql_subscription] macro expansion.
• Fix memory leak caused by error handling in GraphQL subscriptions.
• Make juniper::ValuesStream to support error batching.

Checklist

• Tests are added
• Documentation is added (including Book)
• CHANGELOG is filled

[50U10FCA7]

FCM

Fix errors handling in GraphQL subscriptions (#1371)

- support panic handling in `juniper_graphql_ws` crate
- fix errors mapping in `#[graphql_subscription]` macro expansion in `juniper_codegen` crate
- fix memory leak caused by incorrect errors handling in GraphQL subscriptions machinery
- refactor `juniper::ValuesStream` to support error batching in `juniper` crate