Add filename when invalid metadata in page

[lord63]

problem

when invalid metadata in page, simiki g will failed with the following error message:

Traceback (most recent call last):
  File "path/venv/bin/simiki", line 11, in <module>
    sys.exit(main())
  File "path/venv/lib/python2.7/site-packages/simiki/cli.py", line 404, in main
    generator.generate(include_draft=args['--draft'])
  File "path/venv/lib/python2.7/site-packages/simiki/cli.py", line 181, in generate
    self.generate_tags()
  File "path/venv/lib/python2.7/site-packages/simiki/cli.py", line 217, in generate_tags
    meta, _ = g.get_meta_and_content(do_render=False)
  File "path/venv/lib/python2.7/site-packages/simiki/generators.py", line 135, in get_meta_and_content
    meta_str, content_str = self.extract_page(self._src_file)
  File "path/venv/lib/python2.7/site-packages/simiki/generators.py", line 203, in extract_page
    raise Exception('extracting page with format error, '
Exception: extracting page with format error, see <http://simiki.org/docs/metadata.html>

users may be confused because they don't know which page has format error. Add filename in the error message could be more friendly.

How to reproduce

simiki version: Simiki 1.6.2.3

page content:

---
title: "Meta Data"
date: 2013-10-12 00:00

hello

running command simiki g will get the exception.