Multilogistic: Return error if all observations in category has missing data

Pivotal Tracker: 60643012

Changes:
    - For mlogit and all estimators for mlogit, we find the all categories
    in data and subtract from this set all categories with at least 1 non-missing
    data observation. If this set-difference is not empty, then an error is
    returned.
    - Also fixed a problem in Python AL for validation of arguments.