add missing file error

cache.rkt

@@ -37,6 +37,8 @@
     (with-handlers ([exn:fail? (λ(exn) (error (format "cached-require: ~a is not a valid path" path-string)))])
       (->complete-path path-string)))  
   
+  (when (not (file-exists? path)) (error (format "cached-require: ~a does not exist" (path->string path))))
+  
   (when (or (not (cache-has-key? path))
             (> (file-or-directory-modify-seconds path) (hash-ref (cache-ref path) 'mod-time)))
     (cache path))