Alert the user when we don't have permission to access the page with the selection

When the Open Selected Links context-menu item is used on a restricted page (e.g. AMO, or a browser/extension page) there's an uncaught permission exception ("Error: Missing host permission for the tab or frames") and nothing happens. OSL should catch the exception and let the user know somehow.

We might still be able to get the plain text of the selection, and at least open text links that way.