Lua: fix peeker for PandocError.

String error messages were incorrectly popped of the stack when
retrieving a PandocError.

pandoc-lua-engine/src/Text/Pandoc/Lua/Marshal/PandocError.hs

@@ -46,5 +46,7 @@ peekPandocError idx = Lua.retrieving "PandocError" $
   liftLua (Lua.ltype idx) >>= \case
     Lua.TypeUserdata -> peekUD typePandocError idx
     _ -> do
-      msg <- liftLua $ Lua.state >>= \l -> Lua.liftIO (Lua.popErrorMessage l)
+      msg <- liftLua $ do
+        Lua.pushvalue idx
+        Lua.state >>= \l -> Lua.liftIO (Lua.popErrorMessage l)
       return $ PandocLuaError (UTF8.toText msg)

pandoc-lua-engine/src/Text/Pandoc/Lua/PandocLua.hs

@@ -88,9 +88,9 @@ instance PandocMonad PandocLua where
 -- | Retrieve a @'PandocError'@ from the Lua stack.
 popPandocError :: LuaE PandocError PandocError
 popPandocError = do
-  errResult <- runPeek $ peekPandocError top
+  errResult <- runPeek $ peekPandocError top `lastly` pop 1
   case resultToEither errResult of
-    Right x -> return x
+    Right x  -> return x
     Left err -> return $ PandocLuaError (T.pack err)
 
 -- | Conversions between Lua errors and 'PandocError' exceptions.