"in a productive position" is reported instead of "type name is unbound"

A repro:

```
#lang typed/racket/base

(define-type Foo (Rec z (U (List Any) (Bar z))))
```
It generates an error:

```
Type Checker: parse error in type;
 in a productive position
  variable: z in: z
```

I would expect something like:

```
 Type Checker: parse error in type;
 type name `Bar' is unbound in: (Bar z)
```