emacs: Add 'guix-find-file'.

* emacs/guix-utils.el (guix-find-file): New procedure.
* emacs/guix-info.el (guix-file): Use it.

emacs/guix-info.el

@@ -334,7 +334,7 @@ VAL is a list, call the function on each element of this list."
   'face 'guix-info-file-path
   'help-echo "Find file"
   'action (lambda (btn)
-            (find-file (button-label btn))))
+            (guix-find-file (button-label btn))))
 
 (define-button-type 'guix-url
   :supertype 'guix

emacs/guix-utils.el

@@ -154,6 +154,12 @@ accessed with KEYS."
     (dolist (key keys val)
       (setq val (cdr (assq key val))))))
 
+(defun guix-find-file (file)
+  "Find FILE if it exists."
+  (if (file-exists-p file)
+      (find-file file)
+    (message "File '%s' does not exist." file)))
+
 
 ;;; Diff