Fix embarrassing bug in getWikiPageList

* That function was referring to a variable that didn't exist in
  that scope at all.
* Also, the API was bad. Fix the API and implementation so that
  getWikiPageList returns errors instead of serving an error page.
Improvements in error handling and data flow

* Make the wikiPage filter read the available wiki pages and make
  them available through the request object as other things are.
* Also, make wikiPage take care of the errors to simplify the code
  that shows and creates pages.
Make page creation aware of the available wiki pages

* This is much easier now that the wikiPage filter exposes that
  information and takes care of the errors.
* This improves the page creation a little bit, as links to wiki
  pages that don't exist (yet?) are recognised and shown as such.