redex-check with broken `#:prepare` fails silently.

[florence]

In the program:

#lang racket
(require redex)
(define-language L)
(redex-check L natural (number? (term natural)) #:prepare (lambda _ 'x))

Produces redex-check: no counterexamples in 1000 attempts.

I would have expected either an error or a complaint about generation failures (because x is not a natural).