bugfix: pandoc error message was lost and generator failed

Template render error: TypeError: Cannot call method 'replace' of
undefined
actually means that pandoc refused to create the document, but original
pandoc's explanation was lost. Now it gets printed to console
even better error handling, wintersmith displays original pandoc error message
Merge pull request #10 from xmojmr/master

bugfix: pandoc error message was lost and generator failed