HTTP Service: 404 page shows registered servlets paths

404 and 500 pages are now generated by the HTTP Service, not the request handler.
A service is now able to customize 404 and 500 error pages

Specification: pelix.http.error.pages
Methods:
- make_exception_page(path, str_stack)
- make_not_found_page(path)
=> both returns a HTML page as a string