[FIX] ensure default addon paths are valid (existing)

User-provided addons paths are checked for existence (and rejected), but
default addons paths are not checked, and blow up when trying to listdir
them (e.g. when http.py tries to load modules).

This is an issue when using Odoo from the distributed tarballs, because
the packaging currently moves all modules to openerp/addons and removes
the root ("main") addons directory.