The adapter aims to:
- simplify the common case of synchronous resolution to a single error
- support ThreadLocal context propagation on an opt-in basis
This replaces the SyncDataFetcherExceptionResolver and removes the need
to propagate ThreadLocal context to every resolver.
The errorType field on GraphQLError is specific to GraphQL Java and when
serialized via GraphqlErrorHelper#toSpecification it becomes an
extension with the key "classification".
This commit ensures that we correctly deserialize back to errorType from
the "classification" extension but unfortunately that's limited to the
ErrorClassification enums that we know of.
