Fix deprecations in python-gitlab, improve error handling

the list method on some gitlab objects used to return a python list, or an iterator if passed as_list=False. The as_list argument was deprecated in favor of a new iterator argument, which defaults to False.

With this change, the list by default returns a python list of 20 elements maximum. If more than 20 elements are available via the API, they will be dropped unless list is passed iterator=True