Lizzy traffic fails when a rolled back stack exists

[danielkarch]

We use lizzy as part of our Jenkins setup and keep running into a problem. When we want to get the current traffic distribution via lizzy traffic OUR_APP, lizzy fails with

Requesting traffic info.. EXCEPTION OCCURRED: 404 Client Error: NOT FOUND for url: https://lizzy-ourteam.ourteam.zalan.do/api/stacks/ourapp-someOldStack/traffic

[AGENT] Stack not found: ourapp-someOldStack

where ourapp-someOldStack is a stack that has been rolled back, but not deleted. When we delete the stack the error goes away, but the presence of such a stack should not cause lizzy to fail.