data: Fix problems with using non-existing metadata on a linked page. The page data wrapper now has supports for loader to raise a custom exception to declare that they can't return a value after all (this is especially useful for the wildcard loader). The returned value, if any, is also now cached properly.

#!/bin/sh

CUR_DIR="$( cd "$( dirname "$0" )" && pwd )"
CHEF=${CUR_DIR}/../bin/chef
OUT_DIR=${CUR_DIR}/../piecrust/resources/messages
ROOT_DIR=${CUR_DIR}/messages

$CHEF --root=$ROOT_DIR bake -o $OUT_DIR
rm ${OUT_DIR}/index.html