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.
filepath was used inconsistently throughout the hugolib. With the
introduction of source and target modules, all path are normalized to
"/". This simplifies the processing of paths. It does mean that
contributors need to be aware of using path/filepath in any module other
than source or target is not recommended. The current exception is
hugolib/config.go
