Wrong info message when uninstalling an extension from a subwiki

Originally reported by Raluca Moisa.

Steps to reproduce:

• Install a new extension on the current wiki (for example I used "Contrib JIRA Macro")
• Go to the Installed Extensions tab in the administration page and click on the Uninstall button for your extension

You'll get an info message (uninstall.png) that the extension could not be found. Despite this message the extension is properly uninstalled. If instead of Uninstall you click on "Uninstall from Farm" you get the correct info message (uninstallFromFarm.png).