Allowed Contributors plug-in fails with KeyError

[dobey]

If a person or team on launchpad is inaccessible via the API for whatever reason (nonexistent, private, etc…), then attempting to access them via launchpad.people[person_or_team] results in a KeyError: 'person_or_team' being raised. This is a very uninformative error, and gets raised to the top, causing tarmac to exit, and other branches to not be examined.