add originalError

[JoviDeCroock]

So I think this is better than relying on the Error instance received from the backend having an originalError property this way we allow for our users to have enriched errors with for instance code or key.

In the docs they also state this to be an Error instance so I think this is perfectly valid https://graphql.org/graphql-js/error/#graphqlerror

Resolves: #469

[kitten]

Neat! I didn’t realise that we could just point at the raw serialised error here, but that makes complete sense 👌