Print template parsing errors to aid troubleshooting

Added a new Template.PrintErrors() function call,
used in hugolib/site.go#Process() so it does not clutter
up `go test -v ./...` results.

Special thanks to @tatsushid for mapping out the call trace
which makes it a lot easier to find the appropriate places
to place the Template.PrintErrors() call.

Fixes #316

hugolib/site.go

@@ -269,6 +269,7 @@ func (s *Site) Process() (err error) {
 		return
 	}
 	s.prepTemplates()
+	s.Tmpl.PrintErrors()
 	s.timerStep("initialize & template prep")
 	if err = s.CreatePages(); err != nil {
 		return