catch networking errors and show a dialog

that doesn't say "internal error"

closes PR 14254

pkgs/drracket-pkgs/drracket/drracket/private/frame.rkt

@@ -536,29 +536,31 @@
   ;; install-plt-from-url : string (union #f dialog%) -> void
   ;; downloads and installs a .plt file from the given url
   (define (install-plt-from-url s-url parent)
-    (define-values (port size)
-      (let-values ([(port header)
-                    (get-pure-port/headers (string->url s-url)
-                                           #:redirections 5)])
+    (define url (string->url s-url))
+    (define-values (port header) 
+      (with-handlers ([exn:fail? (λ (x) (values #f x))])
+        (get-pure-port/headers url #:redirections 5)))
+    (cond
+      [port
        (define size
          (let* ([content-header (extract-field "content-length" header)]
                 [m (and content-header
                         (regexp-match "[0-9]+" content-header))])
            (and m (string->number (car m)))))
-        (values port size)))
-    (let* ([tmp-filename (make-temporary-file "tmp~a.plt")]
-           [header (purify-port port)]
-           [d (make-object dialog% (string-constant downloading) parent)] 
-           [message (make-object message% (string-constant downloading-file...) d)] 
-           [gauge (if size 
+       (define tmp-filename (make-temporary-file "tmp~a.plt"))
+       (define header (purify-port port))
+       (define d (make-object dialog% (string-constant downloading) parent))
+       (define message (make-object message% (string-constant downloading-file...) d))
+       (define gauge (if size 
                          (make-object gauge% #f 100 d) 
-                      #f)] 
-           [exn #f] 
+                         #f))
+       (define exn #f)
        ; Semaphores to avoid race conditions: 
-           [wait-to-start (make-semaphore 0)] 
-           [wait-to-break (make-semaphore 0)] 
+       (define wait-to-start (make-semaphore 0))
+       (define wait-to-break (make-semaphore 0))
        ; Thread to perform the download: 
-           [t (thread 
+       (define t
+         (thread 
           (λ () 
             (semaphore-wait wait-to-start) 
             (with-handlers ([exn:fail?
@@ -580,7 +582,7 @@
                           (display s) 
                           (loop (+ total (string-length s))))))))
                 #:mode 'binary #:exists 'truncate))
-                 (send d show #f)))]) 
+            (send d show #f))))
        (send d center) 
        (make-object button% (string-constant &stop)
          d
@@ -615,7 +617,16 @@
           (parameterize ([error-display-handler drracket:init:original-error-display-handler])
             (run-installer tmp-filename
                            (λ ()
-                            (delete-file tmp-filename))))])))
+                             (delete-file tmp-filename))))])]
+      [else
+       (define exn header)
+       (define sp (open-output-string))
+       (parameterize ([current-error-port sp])
+         (drracket:init:original-error-display-handler (exn-message exn) exn))
+       (message-box (string-constant drracket)
+                    (string-append
+                     (string-constant install-plt-error-downloading)
+                     (get-output-string sp)))]))
   
   (define keybindings-frame%
     (class frame%

pkgs/string-constants-pkgs/string-constants-lib/string-constants/private/english-string-constants.rkt

@@ -462,6 +462,9 @@ please adhere to these guidelines:
  (install-plt-file-tab "File")
  (install-plt-filename "Filename:")
  (install-plt-url "URL:")
+ ; an error message from a primitive operation is appended to the end of this message.
+ (install-plt-error-downloading "There was an error when downloading the"
+                                " .plt file.\n\nDetails:\n")
  (install-plt-error-header "There was an error when checking the validity of the downloaded .plt file. Please check the url and try again.")
  
  ;; install plt file when opened in drscheme strings