adjust htdp so missing teachpacks just print error messages

and continue, rather than raising exceptions

pkgs/drracket/drracket/private/main.rkt

@@ -110,14 +110,7 @@
 
 (drr:set-default 'drracket:htdp:last-set-teachpacks
                  '() 
-                         (λ (x)
-                           (and (list? x)
-                                (andmap (λ (x)
-                                          (and (list? x)
-                                               (pair? x)
-                                               (eq? (car x) 'lib)
-                                               (andmap string? (cdr x))))
-                                        x))))
+                 (listof (cons/c 'lib (listof string?))))
 (drr:set-default 'drracket:defs/ints-horizontal #f boolean?)
 
 (drr:set-default 'drracket:child-only-memory-limit (* 1024 1024 128)

pkgs/htdp/lang/run-teaching-program.rkt

@@ -98,12 +98,16 @@
 ;; syntax objects that require them (tagged
 ;; with stepper-skip-completely)
 (define (teachpacks->requires teachpacks)
+  (filter
+   values
    (for/list ([tp (in-list teachpacks)])
-    (unless (file-exists? (build-path (apply collection-path (cddr tp))
+     (cond
+       [(file-exists? (build-path (apply collection-path (cddr tp))
                                   (cadr tp)))
-      (error 'teachpack (missing-tp-message tp)))
         (stepper-skip
-     (datum->syntax #f `(require ,tp)))))
+         (datum->syntax #f `(require ,tp)))]
+       [else
+        (eprintf "~a\n" (missing-tp-message tp))]))))
 
 (define (missing-tp-message x)
   (let* ([m (regexp-match #rx"/([^/]*)$" (cadr x))]