It worked for a time since we did list some gitlab instances (T1139).
Investigate and fix.
```
Jun 29 07:34:04 worker01 python3[18335]: [2019-06-29 07:34:04,108: ERROR/ForkPoolWorker-3] Task swh.lister.gitlab.tasks.FullGitLabRelister[25599a68-d227-4d2f-9e94-7345ec7b0d2d] raised unexpected: ValueError('Problem during information fetch: 404',)
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/celery/app/trace.py", line 382, in trace_task
R = retval = fun(*args, **kwargs)
File "/usr/lib/python3/dist-packages/swh/scheduler/task.py", line 45, in __call__
return super().__call__(*args, **kwargs)
File "/usr/lib/python3/dist-packages/celery/app/trace.py", line 641, in __protected_call__
return self.run(*args, **kwargs)
File "/usr/lib/python3/dist-packages/swh/lister/gitlab/tasks.py", line 47, in full_gitlab_relister
_, total_pages, _ = lister.get_pages_information()
File "/usr/lib/python3/dist-packages/swh/lister/gitlab/lister.py", line 75, in get_pages_information
'Problem during information fetch: %s' % response.status_code)
ValueError: Problem during information fetch: 404
```