include link to desktopSharing extension when install fails

When jitsi-meet is running in another webserver/url, it can't automatically install the extension, since the website isn't a verified webstore site for the extension (and never should be).

The original error message on the install button doesn't provide a resolution to the user. This very simple patch adds a link to the message, providing a way to the user install the extension directly through the webstore.