friendlier error for DOIs without citation metadata

[kevinushey]

When inserting a citation from a DOI, the lookup uses DOI content negotiation (an Accept: application/vnd.citationstyles.csl+json header against https://doi.org/<doi>). For DOIs whose registration agency does not support content negotiation (e.g. 10.1000/182, the DOI Handbook itself), the request just redirects to the HTML landing page, and the editor surfaced a raw JSON parse error:

invalid json response body at https://www.doi.org/the-identifier/resources/handbook/ reason: Unexpected token '<', "<!DOCTYPE "... is not valid JSON

This PR detects the "resolved, but not to JSON" case and reports a human-readable message instead:

This DOI was found, but no citation data is available for it. This usually means the organization that registered the DOI does not support automatic citation lookup. To cite this work, you can add an entry to your bibliography manually.

Noticed while reviewing #1060 (#1060 (review)).

Verified manually by calling doiServer().fetchCSL() directly: 10.1000/182 now returns the friendly error, and a real Crossref DOI (10.1038/nature12373) still returns CSL metadata.

[posit-snyk-bot]

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