[nfss] Getting data from unexisting project returns empty tests

[Leo Arias]

When I query for the data points of an unexisting project, I get a json back with no tests.
For example, http://nfss.staging.ubuntu.com/api/v1/idontexist returns:

{"project_name": "idontexist", "tests": {}}

I think that is should return an error instead, to make it clear that it's different a project that doesn't exist from a project that exists but has no tests nor data.