`Person` can fail for test users or course accounts

I found two more cases that fail. As test users and course accounts they don't have all fields required to populate Person. Similar to #36

The cli works because it only returns the json object. I assume there is no good way to know which elements are hard requirements in IAM

import ethz_iam_webservice

e = ethz_iam_webservice.login(IAM_USERNAME, IAM_PASSWORD)

e.person.get_person("iac25")
e.person.get_person("iac-user")