Manual filter list update broken

Environment

adblockpluschrome "next" branch

Steps to reproduce

1. Open Advanced tab in options page
2. Click on "Update all filter lists" button

Observed behavior

Nothing happens in UI and error is shown in extension's background page.

Expected behavior

Filter lists are updated and no error is shown in extension's background page.

Further information

This issue occurs because we missed an occurrence of filterStorage.subscriptions in the code when working on #219 (closed).