Prevent adding unsupported links from the web page context menu

[aecreations]

The context menu item appears even on system pages (e.g. "about:debugging"). There doesn't seem to be a way to control visibility of an extension's context menu items for specific URL protocols.

Prevent the user from adding unsupported URLs to the reading list by checking the URL that is being added from the page context menu. If the page shouldn't be added to the reading list, show a notification with an error message, e.g.: "This page can't be added to Read Next."