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Newsletter extension allows to manage and subscribe to newsletters on MediaWiki.

The extension adds a Special:Newsletters page which lists all the newsletters available on the wiki. This page also allows users to subscribe and unsubscribe from newsletters by clicking on a link on this page. There is a JavaScript click handler which lets the user subscribe/unsubscribe with one-click without navigating away from the page.

Currently, the click handler does not handle errors. The API already returns errors so your task is to add error handling code to this event handler.

See https://phabricator.wikimedia.org/T115874 for details.