Fails to serialize and deserialize private types

```
type private Person = 
{
    firstName: string
    lastName: string
}

let x = 
{
    firstName = "John"
    lastName = "Doe"
}

Json.serialize x
```

The above snippet throws an error saying that the type is not a Record. It would be great if the error message could identify that the type is inaccessible, or suggest making sure that the type is publicly scoped.
