Improve sudo/kdesu error message

Improve sudo/kdesu error message

Now that you can get elevated privileges while using the app, we can
display a more useful and descriptive error message when people try to
run Dolphin using `sudo` or `kdesu`.

Also bump the frameworks dependency to 5.90 since that's the KIO
version that includes this change, so we can be sure that the message is
always accurate.