Server: return a 4xx code on metadata request for non-existing machines.

[epa095]

Requesting the metadata for a non-existing machine gives a 200 result with the following metadata:

{"gordo-server-version": "0.48.3", 
"metadata": {}, 
"env": {"MODEL_COLLECTION_DIR": "/gordo/models/ep-tp-483/1580813025723"}, 
"revision": "1580813025723"}

It should probably return a 4xx code to indicate that it is a non-existing machine. Example;

curl 127.0.0.1:8000/gordo/v0/ep-tp-483/non-exdsdf/metadata