error.rs: Add an empty RsvgError and impl glib::error:ErrorDomain for it
error.rs: Add an empty RsvgError and impl glib::error:ErrorDomain for it

We will use this to call glib::Error::new().  Unfortunately we don't
have detailed error codes in the public API; maybe this can change later.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg