Handling AttributeError on structured citation prompt failure

[jamesbraza]

Our ParsingSettings.structured_citation_prompt extracts a JSON that is supposed to contain the paper's title, DOI, and authors. However, last night I hit a new crash possibility:

1. The paper Casein Kinase 1δ Triggers Giant Ankyrin Expression had its references on the first page.
2. This led to ParsingSettings.structured_citation_prompt extracting a list of authors dicts (one dict per reference).
3. list[dict] was not accounted for in Docs.aadd, it leads to an AttributeError and crashes the indexing process.

This PR handles the AttributeError possibility, and logs a warning if it happens

[mskarlin]

thanks for addressing the comment