Add __Service

Author: martinbonnin

rfcs/OnError.md

@@ -41,13 +41,21 @@ In the end state where error propagation is disabled and clients are error-handl
 The default error propagation is indicated in introspection with a new meta-field, `onErrorDefault` of `String!` type.
 
 ```graphql
-extend type __Schema {
+type __Service {
     """
     Indicates the error behaviour when `onError` is not present in the request.
     One of `NULL`, `PROPAGATE`, `HALT`
     """
     onErrorDefault: String!
 }
+extend type __Schema {
+    """
+    Information relative to the service.
+    In a perfect world, this is a separate meta-field separate from __Schema but new meta fields are not discoverable
+    and this is added as a `__Schema` field for backward compatibility reasons.
+    """
+    service: __Service
+}
 ```
 
 Services not supporting `onError` must not support `onErrorDefault` and must ignore the `onError` request property.