Type exported as JS breaks imports (v15.x.x branch)

@enisdenjo This export of GraphQLFormattedError should be prefaced with the type keyword to avoid it exporting as regular JS in your build process: https://github.com/graphql/graphql-js/blob/v15.10.0/src/error/formatError.js
`export { formatError, GraphQLFormattedError } from './GraphQLError';` in src/error/formatError.js of the v15 branch for the link-averse. 
