Add 404 responses in the case of non-existent artifacts

This basically just cleans up the behavior of the rest interface. So for example, instead of return an empty list for the results of a non-existent calculation, we now return a 404.

https://github.com/gem/oq-engine-server/pull/7 should merge first for a clean diff.

This is helpful for https://bugs.launchpad.net/oq-platform/+bug/1216832.