FIX TocGenerator to accept attributed headings

- TocGenerator skips any headings with attributes (via RE)
- an attribute may be added using AttrList extension
- a skipped heading was not being displayed in ToC
- now it is :)
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
tell which key is invalid schema
add 'Go back' link to error page
Merge pull request #73 from jangxyz/ADD-kinder-error-page

Add kinder error page
Merge branch 'master' of https://github.com/akngs/ecogwiki
Remove "print"
Merge pull request #71 from jangxyz/FIX-attrlist-header

FIX TocGenerator to accept attributed headings
Merge branch 'master' of https://github.com/akngs/ecogwiki
Merge pull request #72 from jangxyz/FIX-kinder-toc-error

FIX heading error message to provide more info
Merge branch 'master' of https://github.com/akngs/ecogwiki
Merge branch 'master' into struct-editor