Inform user about errors that occur while using the options page

Background

In eyeo/specs/spec#139 we discussed when to change the state of checkboxes and toggle buttons since those trigger asynchronous actions which can fail. In that context, we found that no matter how we end up doing it, the user won't know why an action failed. That is why we should expose this information to the user.

Goal

Visually expose when there has been an error in subscribing to any filter list within the General tab.

What to do

Implement the error message as specified in https://gitlab.com/eyeo/specs/spec/merge_requests/169