Class: Warn instead or erroring if we can't fetch media

If deferred media can't be fetched, we catch the error and warn
instead. We add an internal function for fetching which returns a Maybe
value, and then run catMaybes to only keep the Just's.

src/Text/Pandoc/Class.hs

@@ -102,6 +102,7 @@ import System.IO.Error
 import qualified Data.Map as M
 import Text.Pandoc.Error
 import Data.Monoid
+import Data.Maybe (catMaybes)
 
 class (Functor m, Applicative m, Monad m, MonadError PandocError m)
       => PandocMonad m where
@@ -188,15 +189,27 @@ instance Monoid DeferredMediaBag where
   mappend (DeferredMediaBag mb lst) (DeferredMediaBag mb' lst') =
     DeferredMediaBag (mb <> mb') (lst <> lst')
 
+    
+-- the internal function for downloading individual items. We want to
+-- catch errors and return a Nothing with a warning, so we can
+-- continue without erroring out.
+fetchMediaItem :: PandocMonad m
+               => DeferredMediaPath
+               -> m (Maybe (FilePath, B.ByteString, Maybe MimeType))
+fetchMediaItem dfp =
+  (do (bs, mbmime) <- downloadOrRead Nothing (unDefer dfp)
+      return $ Just $ (unDefer dfp, bs, mbmime))
+  `catchError`
+  (const $ do warning ("Couldn't access media at " ++ unDefer dfp)
+              return Nothing)
+
 fetchDeferredMedia' :: PandocMonad m => m MediaBag
 fetchDeferredMedia' = do
   (DeferredMediaBag mb defMedia) <- getsCommonState stDeferredMediaBag
-  fetchedMedia <- mapM (\dfp -> downloadOrRead Nothing (unDefer dfp)) defMedia
+  fetchedMedia <- catMaybes <$> mapM fetchMediaItem defMedia
   return $ foldr
-    (\(dfp, (bs, mbMime)) mb' ->
-       MB.insertMedia (unDefer dfp) mbMime (BL.fromStrict bs) mb')
-    mb
-    (zip defMedia fetchedMedia)
+    (\(fp, bs, mbMime) mb' -> MB.insertMedia fp mbMime (BL.fromStrict bs) mb')
+    mb fetchedMedia
 
 fetchDeferredMedia :: PandocMonad m => m ()
 fetchDeferredMedia = fetchDeferredMedia' >>= setMediaBag