Add overwrite_html flag to to_html.produce_html_for_modules

Enable this flag means that the index.html will be removed before a new
one is generated.
Stop raising a plpy error when produce_html_for_module returns a message

produce_html_for_module now raises an exception if there's an error.
Merge pull request #156 from karenc/remove-index-html

Overwrite index html with --force