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.

import os


def slashfix(path):
    if path is None:
        return None
    if isinstance(path, str):
        return path.replace('/', os.sep)
    fixed = []
    for p in path:
        fixed.append(p.replace('/', os.sep))
    return fixed