check-exn: stop ignore message argument

[bennn]

Fix check-exn to print the user's message if they supplied one, rather than the generic "No exception raised".

I think I need #59 to test this. Until then here's some checks I'd use to test:

#lang racket/base
(require rackunit)

(check-not-exn (λ () (/ 1 0)) "don't divide by 0")

(check-exn exn:fail? (λ () (void)) "this should raise an exception")

(check-exn exn:fail:user? (λ () (error 'death)) "should be user exn")

All these should print a message: with the user's message rather than the generic ones.
(Racket 6.7 does this, Racket 6.10 candidate doesn't)