make --json-error-messages capture url download errors

Convert Utility.Url to return Either String so the error message can be
displated in the annex monad and so captured.

(When curl is used, its errors are still not caught.)

Remote/BitTorrent.hs

@@ -206,7 +206,7 @@ downloadTorrentFile u = do
 					withTmpFileIn othertmp "torrent" $ \f h -> do
 						liftIO $ hClose h
 						ok <- Url.withUrlOptions $ 
-							liftIO . Url.download nullMeterUpdate u f
+							Url.download nullMeterUpdate u f
 						when ok $
 							liftIO $ renameFile f torrent
 						return ok