FIX heading error message to provide more info

change:

- add `toc_generator.is_invalid` method to return error msg and
- `wiki_page.validate_new_content` calls `is_invalid` instead of `validate`
- and use the reason returned for error message
- `toc_generator..validate` is left for backward compatibility, if any

provides:

- 'Headings should start from H1' returns explicit error heading
- 'Invalid level of headings' tells you which and why
- 'Duplicate paths not allowed' shows you which one
Merge pull request #72 from jangxyz/FIX-kinder-toc-error

FIX heading error message to provide more info