Fix several Vala warnings
[CCode (finish_function = "...")] is deprecated, use [CCode (finish_name = "...")] instead.
finish_function is deprecated since a long time now.
warning: DBus methods are recommended to throw at least `GLib.Error' or `GLib.DBusError, GLib.IOError'
This has always been an issue, the compiler warns since a few years now that this is required.