Error message for `range` is leaky

The code `range([1,2])`
results in the error
```
range: contract violation
  expected: num?
  given: [1, 2]
  in: the 1st argument of
      the 3rd case of
      (case->
       (-> num? num? num? AnyC)
       (-> num? num? AnyC)
       (-> num? AnyC))
```

This is the correct contract for DSSL's range, but having an error message in terms of `case->` and prefix `->` isn't ideal.