Prevent scarry error at startup (pyinotify ERROR ... ENOENT)

If the clipboard file doesn't exist (and it doesn't at system startup),
pyinotify will output an error to stderr. This situation is already
handled by also monitoring the directory itself for CREATE event. But
prevent error message.

qubesmanager/main.py

@@ -2057,8 +2057,9 @@ def main():
                  EventsCodes.OP_FLAGS.get('IN_MODIFY'))
     wm.add_watch(os.path.dirname(system_path["qubes_store_filename"]),
                  EventsCodes.OP_FLAGS.get('IN_MOVED_TO'))
-    wm.add_watch(qubes_clipboard_info_file,
-                 EventsCodes.OP_FLAGS.get('IN_CLOSE_WRITE'))
+    if os.path.exists(qubes_clipboard_info_file):
+        wm.add_watch(qubes_clipboard_info_file,
+                     EventsCodes.OP_FLAGS.get('IN_CLOSE_WRITE'))
     wm.add_watch(os.path.dirname(qubes_clipboard_info_file),
                  EventsCodes.OP_FLAGS.get('IN_CREATE'))
     wm.add_watch(os.path.dirname(table_widgets.qubes_dom0_updates_stat_file),