Support for new Relay @catch and @throwOnFieldError directives added in v18

[kawazoe]

With the recent drafts for Semantic Nullability in the GraphQL spec, Relay added features related, but not dependent, to this new specification. Namely, the @catch directive and the @throwOnFieldError directive. These new directives should be added to the relay schema provided by this extension as they are considered stable in Relay.

See #736 for Semantic Nullability support.