Custom writer: Raise PandocLuaException instead of using 'error'.

Eventually we'll change the return type so that no exception
is involved, but at least this can be trapped.

src/Text/Pandoc/Writers/Custom.hs

@@ -1,5 +1,5 @@
 {-# LANGUAGE OverlappingInstances, FlexibleInstances, OverloadedStrings,
-    ScopedTypeVariables #-}
+    ScopedTypeVariables, DeriveDataTypeable #-}
 {-# OPTIONS_GHC -fno-warn-orphans #-}
 {- Copyright (C) 2012-2014 John MacFarlane <jgm@berkeley.edu>
 
@@ -35,6 +35,7 @@ import Text.Pandoc.Definition
 import Text.Pandoc.Options
 import Data.List ( intersperse )
 import Data.Char ( toLower )
+import Data.Typeable
 import Scripting.Lua (LuaState, StackValue, callfunc)
 import Text.Pandoc.Writers.Shared
 import qualified Scripting.Lua as Lua
@@ -43,6 +44,7 @@ import Data.ByteString (ByteString)
 import qualified Data.ByteString.Char8 as C8
 import Data.Monoid
 import Control.Monad (when)
+import Control.Exception
 import qualified Data.Map as M
 import Text.Pandoc.Templates
 
@@ -146,6 +148,11 @@ instance StackValue Citation where
   peek = undefined
   valuetype _ = Lua.TTABLE
 
+data PandocLuaException = PandocLuaException String
+    deriving (Show, Typeable)
+
+instance Exception PandocLuaException
+
 -- | Convert Pandoc to custom markup.
 writeCustom :: FilePath -> WriterOptions -> Pandoc -> IO String
 writeCustom luaFile opts doc@(Pandoc meta _) = do
@@ -156,7 +163,7 @@ writeCustom luaFile opts doc@(Pandoc meta _) = do
   -- check for error in lua script (later we'll change the return type
   -- to handle this more gracefully):
   when (status /= 0) $
-    Lua.tostring lua 1 >>= error
+    Lua.tostring lua 1 >>= throw . PandocLuaException
   Lua.call lua 0 0
   -- TODO - call hierarchicalize, so we have that info
   rendered <- docToCustom lua opts doc