Revise the deprecation strategy

Git users and theme authors two Hugo releases to fix:

1. With a visible warning
2. Then with an ERROR that exits with -1

Fixes #2726

hugolib/page_test.go

@@ -1270,6 +1270,7 @@ func TestIndexPageSimpleMethods(t *testing.T) {
 	} {
 
 		n := &Page{pageInit: &pageInit{}, Kind: KindHome}
+
 		n.RSSLink = "rssLink"
 
 		if !this.assertFunc(n) {