`check-exn` gives internal error message when give the wrong number of arugments

```
#lang racket

(require rackunit)
(check-exn (lambda () 'hi))
```
Errors with:

```
rackunit-lib/rackunit/private/check.rkt:112:13: application: no case matching 1 non-keyword argument
  procedure: check-impl
  arguments...:
```

Which 1. doesn't talk about check-exn, 2. points into the internals of rackunit.