TypeScript error in handleErrors
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### Summary

The [handleError docs](https://www.graphile.org/postgraphile/usage-library/#handleerrors) state "This function will be passed the list of errors that occurred during the GraphQL operation (if any occurred) and must return this same list, or a list that will be used to replace the errors."

But returning the same list results in a TypeError:

```
The type 'readonly GraphQLError[]' is 'readonly' and cannot be assigned to the mutable type 'GraphQLErrorExtended[]'
```

It seems that the `extensions` property is required.
