Return 404 code/status on NotFound

Currently tasks.py is returning '200 OK' on NotFound exceptions, and I figured we want to be compliant from the start by returning a 404 :-)

I used the docs for Pyramid 1.3 [1] for the tweak, but I'm not certain if there's a better way to do this.

[1] http://docs.pylonsproject.org/projects/pyramid/en/1.3-branch/narr/renderers.html#varying-attributes-of-rendered-responses