As of now, the FileImporter codebase uses mostly legacy API requests:
- We don't set formatversion=2, which means version 1 is used by default. The legacy format is the one with these weird * "main content" fields. And it returns only 1 error and discards the rest.
- We don't set errorformat=plaintext, which means the default bc is used.
- We don't set rvslots=main in the revisions query.
Proof-of-concept patch: https://gerrit.wikimedia.org/r/603567
- Update API parameters as above.
- Log API warnings so we catch future deprecations.