Docx writer: Fixed error message when style file can't be parsed.

src/Text/Pandoc/Writers/Docx.hs

@@ -146,7 +146,8 @@ writeDocx mbRefDocx opts doc@(Pandoc (Meta tit auths date) _) = do
   let styledoc = case findEntryByPath stylepath refArchive >>=
                       parseXMLDoc . toString . fromEntry of
                         Just d  -> d
-                        Nothing -> error $ stylepath ++ "missing in reference docx"
+                        Nothing -> error $ "Unable to parse " ++ stylepath ++
+                                           " from reference.docx"
   let styledoc' = styledoc{ elContent = elContent styledoc ++ map Elem newstyles }
   let styleEntry = toEntry stylepath epochtime $ fromString $ showTopElement' styledoc'
   -- construct word/numbering.xml