Minor Description
Imagine that you wish to check if your local wiki has the HTML Macro installed. You got to the admin ui, you select "installed extensions", you type "HTML" in the search box and submit...
... and you get: "There were no extensions found matching 'html'.[...]"
The reason is because the HTML Macro is part of the "Core" installed extensions. The proposal is to improve the search results when there's no result, by searching in "all extensions" category, with a proper message saying that no extension was found in the selected category but matches were found in other categories.