android: Show an error dialog if we can't get permission for VPNs

android: Show an error dialog if we can't get permission for VPNs

This is either because a third-party VPN app has the always-on feature
enabled, or because the user denied the permission in the system's confirmation
dialog.

If the always-on feature is enabled for a connection of the built-in VPN
client we get an IllegalStateException, for which we show an updated and
clearer error message.