Add more accurate typings to formatError

[cheapsteak]

Related to #2294

This updates the typings for formatError to specify that it accepts an GraphQLError as an argument, and expects a GraphQLFormattedError to be returned

TODO:

• Update CHANGELOG.md with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests (NA?)
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[martijnwalraven]

Thanks for making the typing of formatError more specific, we should definitely stop using Function. It doesn't actually take and return ApolloError however, but instead takes GraphQLError and returns GraphQLFormattedError, similar to formatError from graphql-js: https://github.com/graphql/graphql-js/blob/f205c99c676bb71b9c5c043e574a5c96856fc265/src/error/formatError.js#L18

[cheapsteak]

Ah, thanks for the clarification, I'll update that

[cheapsteak]

Updated, please take another look once you have a chance 🙏

[abernix]

LGTM!