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/page_permalink_test.go

@@ -0,0 +1,9 @@
+package hugolib
+
+import (
+	"testing"
+)
+
+func TestPermalink(t *testing.T) {
+
+}