[feature request] Clarify when redex-check generation fails because #:prepare returns a bad term

[florence]

Sometimes the argument to #:prepare will return a term that doesn't match the input pattern, which causes generation to fail.

Usually, for me at least, this is an error. Ideally it would be nice to be able to configure redex-check to error when this happens, but it might also be good change the message (with Y generation failures) to something like (with X generation failures, Y of which were caused by prepare) or something.