generate-references fails to log error with incorrect PubMed ID

[larsgw]

WP4220 has the "PubMed ID" 9783642403361. This should produce an error.

[larsgw]

The PubMed API returns an empty array for such IDs (i.e. numbers only), presumably because they could be valid in the future. I'd rather have a 404 for now though.