fix(server): onError should return GraphQLFormattedError

[benjie]

The types sent over the wire should be formatted types - specifically errors should be piped through formatError and the types shouldn't imply that they're instanceof GraphQLError - they're just data.

Warning

This is a breaking change if anyone's onError callback is relying on instanceof GraphQLError checks (seems extremely unlikely?), but otherwise it should be safe because GraphQLError conforms to GraphQLFormattedError.

Tip

I don't actually care about formatError being called in core (feel free to revert that), I just care that the type of the onError hook allows a formatted error be returned for consistency, and that the payload type should not imply the errors are instanceof GraphQLError.

Note

The payloads that send ExecutionResult should also be sending FormattedExecutionResult but that seemed like a larger change so I didn't make it.