Refactor layout selection code

The render code path would use a fallback if there was an exception.
This change instead relies on explicit declaration of the layout to use
and includes a check to see if the layout indeed exists before
attempting to render it.

hugolib/node.go

@@ -21,7 +21,7 @@ import (
 type Node struct {
 	RSSlink     template.HTML
 	Site        SiteInfo
-	layout      string
+//	layout      string
 	Data        map[string]interface{}
 	Title       string
 	Description string