etl/gitlab: Handle a Gitlab server issue
After a recent Gitlab server upgrade, the artifact method from ProjectJob started to respond with the wrong error code when the required artifact path did not exist. See https://github.com/python-gitlab/python-gitlab/issues/1186
So, let's treat this error and warn the user that when the
ChunkedEncodingError happens during the get_artifact
helper function,
it should be caused by this error, and stop raising this exception
further.