pypi loader: Analyze existing errors

Now that the first loading is done, take a look at the existing errors.

This is the meta-task, will possibly open new subtask per case.

kibana dashboard: http://kibana0.internal.softwareheritage.org:5601/app/kibana#/dashboard/32632370-c0bd-11e8-8222-07f3ec376cd5

---

Migrated from T1246 (view on Phabricator)