Make safe_html transforms more robuts against dirty html documents.

  - In case of failure of HTMLParser, lxml take under its hand
  the broken html and recover it. Then put back only once
  to HTMLParser again.
 


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@33407 20353a03-c40f-0410-a6d1-a30d3c3de9de