http: Really gracefully handle missing file on /download.

This is a followup to 103a6ec27b.

* src/cuirass/http.scm (url-handler): Remove ‘catch 'system-error’ for
“/download” handler since ‘respond-file’ cannot possibly throw to
'system-error.  Add explicit ‘file-exists?’ call instead.

src/cuirass/http.scm

@@ -1247,13 +1247,9 @@ passed, only display JOBS targeting this SYSTEM."
                    (respond-json-with-error
                     code "Could not find the requested build product."))))
        (if file
-           (catch 'system-error
-             (lambda ()
-               (respond-file file #:ttl %static-file-ttl))
-             (lambda args
-               (if (= ENOENT (system-error-errno args))
-                   (fail 500)                  ;something's wrong: it vanished
-                   (apply throw args))))
+           (if (file-exists? file)
+               (respond-file file #:ttl %static-file-ttl)
+               (fail 500))                     ;something's wrong: it vanished
            (fail 404))))                          ;no such build product
 
     (('GET "machine" name)