ol-docview: Throw an error when trying to open non-existing file

* lisp/ol-docview.el (org-docview-open): Throw an error when file does
not exist.  Creating an empty document buffer is not useful here.

Fixes https://orgmode.org/list/87zgklmd0g.fsf@localhost

lisp/ol-docview.el

@@ -75,7 +75,9 @@
 		   (string-to-number (match-string 2 link)))))
     ;; Let Org mode open the file (in-emacs = 1) to ensure
     ;; org-link-frame-setup is respected.
-    (org-open-file path 1)
+    (if (file-exists-p path)
+        (org-open-file path 1)
+      (error "No such file: %s" path))
     (when page (doc-view-goto-page page))))
 
 (defun org-docview-store-link ()