syntax-color/lexer-contract: make random-test failure declare itself

Random tests that are implciitly injected into a program should
declare themselves as such when they fail. Otherwise, random
crashes are really confusing.

pkgs/syntax-color-pkgs/syntax-color-lib/syntax-color/lexer-contract.rkt

@@ -33,6 +33,21 @@
          (define-values (txt type paren start end) (lexer in))
          (values txt type paren start end 0 #f))]
       [else lexer]))
+  (define initial-state (pseudo-random-generator->vector
+                         (current-pseudo-random-generator)))
+  (with-handlers ([exn:fail?
+                   (lambda (exn)
+                     (raise
+                      (make-exn
+                       (format (string-append "try-some-random-streams:"
+                                              " random testing of lexer failed\n"
+                                              "  lexer: ~e\n"
+                                              "  pseudo-random state: ~s\n"
+                                              "  error message: ~s")
+                               lexer
+                               initial-state
+                               (exn-message exn))
+                       (exn-continuation-marks exn))))])
     (for ([x (in-range 10)])
       (define size (random 100))
       (define (quash-backslash-r c)
@@ -58,7 +73,7 @@
         (cond
          [(equal? type 'eof) #t]
          [(< end size) (loop new-mode end)]
-        [else #f]))))
+         [else #f])))))
 
 (define (end/c start type)
   (cond