Fix a regression that could cause window layout settings to sometimes not be saved during shutdown

src/calibre/gui2/actions/__init__.py

@@ -345,9 +345,5 @@ class InterfaceAction(QObject):
         Called once per plugin when the main GUI is in the process of shutting
         down. Release any used resources, but try not to block the shutdown for
         long periods of time.
-
-        :return: False to halt the shutdown. You are responsible for telling
-                 the user why the shutdown was halted.
-
         '''
-        return True
+        pass

src/calibre/gui2/ui.py

@@ -930,7 +930,8 @@ class Main(MainWindow, MainWindowMixin, DeviceMixin, EmailMixin,  # {{{
         try:
             self.shutdown()
         except:
-            pass
+            import traceback
+            traceback.print_exc()
         self.restart_after_quit = restart
         self.debug_on_restart = debug_on_restart
         if self.system_tray_icon is not None and self.restart_after_quit:
@@ -994,8 +995,7 @@ class Main(MainWindow, MainWindowMixin, DeviceMixin, EmailMixin,  # {{{
             db.commit_dirty_cache()
             db.prefs.write_serialized(prefs['library_path'])
         for action in self.iactions.values():
-            if not action.shutting_down():
-                return
+            action.shutting_down()
         if write_settings:
             self.write_settings()
         self.check_messages_timer.stop()