[#1011] Fix a broken import in the package controller

An exception was not getting caught because of the way this import was
done.
[#1011] base.py: Explicitly re-raise TemplateNotFound

Functions should not implicitly raise exceptions by failing to catch
exceptions raised by functions that they call, they should explicitly
catch and re-raise the exceptions instead.
[#1011] Update some exception handling in template.py

Catch the new ckan.lib.render.TemplateNotFound exception that render()
now raises, not the Genshi exception it used to.