IDFetcher to return error about relay

[F21]

👍 for this. After working with graphql-go for the last few weeks, I feel this is something that's quite necessary.

Because we are switching on resolvedID.Type and resolvedID.ID and using that to fetch the model, if the model fails to fetch, we need to return an error instead.

[F21]

@sogko Any comments on this one?

[sogko]

@F21 Thanks for bringing this up. Actually this does make sense, since we already have a way to return error within graphql-go.

I'll go ahead and submit a PR soon.

Thanks!