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.

@echo off
setlocal

set CUR_DIR=%~dp0
set CHEF=%CUR_DIR%..\bin\chef
set OUT_DIR=%CUR_DIR%..\piecrust\resources\messages
set ROOT_DIR=%CUR_DIR%messages

%CHEF% --root=%ROOT_DIR% bake -o %OUT_DIR%
del %OUT_DIR%\index.html