Looks like `defaultMaskError` is not exported from `grafserv` package
It seems that the `grafserv` package does not expose the `defaultMaskError` function as being stated by the following comment

https://github.com/graphile/crystal/blob/02235c3de89d2d8bcac5673fb0e8f7e10c6621d1/grafast/grafserv/src/index.ts#L135-L142

