Remove invalid handling of HTTP 404

Prior to this commit, we were catching HTTP 404 Not Found responses
and returning `nil`.  This was masking the fact that the URL for the
facts/find endpoint was invalid; also, the server actually just returns
a 200 with an empty array as the payload if you issue a well-formed
query for a node that puppetdb doesn't know about.
Add acceptance test for fact retrieval / inventory service
Fix url for facts terminus `find` method

The URL got broken in a recent changeset; this commit corrects
it to point to `/v2/nodes/<node_name>` rather than just
`/v2/<node_name>`.  Fixes failing acceptance test.