ExtendedError interface no longer works

### What happened?
Updated from v0.10.2 to v0.13.0 and all the error extensions stopped working. The [docs](https://github.com/99designs/gqlgen/blob/v0.10.2/docs/content/reference/errors.md) used to read:
```
The default error presenter will capture the resolver path and use the Error() message in the response. It will also call an Extensions() method if one is present to return graphql extensions.
```

I'm returning errors that implement the old interface:
```golang
type ExtendedError interface {
Extensions() map[string]interface{}
}
```

But the extensions are not part of the response with `gqlgen` v0.13.0.

Are these meant to be implemented differently now? I can't find a comment in the [migration guide](https://gqlgen.com/recipes/migration-0.11/) nor in the new [errors doc](https://gqlgen.com/reference/errors/)

### What did you expect?
Error extensions to keep working

### Minimal graphql.schema and models to reproduce
Don't have one

### versions
- `gqlgen version`? `v0.13.0`
- `go version`? `1.15`
- dep or go modules? `modules`