Fix old 1.3 style django.conf.urls import in tests
Improve error message in ContentItem.plugin call

Include the content type name too, not just the ID.
Fix an error on rendering when a stale ContentType is found.

Stale content types refer to models which are not part of the project,
e.g. a app was removed or a model was renamed. No associated plugin can
be found. The error was already dropped silently, however the logger
statement caused an error. That error is fixed here.
Fix running tests at Django 1.6
Enable tracebacks in runtests.py