Fixing #1649 - NoReverseMatch raised if apphook doesn't exist

Some apps, like cmsplugin-zinnia and cmsplugin_news will raise NoReverseMatch when trying to generate their menus before an apphook is installed. In sampleapp this is solved by adding a try-except like the one in this commit. Placing the try-except in django-cms instead of inside every app would simplify the process of writing plugins and avoid some confusing 500-errors for new users trying to install plugins.