A better error message for when an STD is not found would be helpful

[paulehoffman]

Description

Making a guess about which xi:include to use for an STD: https://bib.ietf.org/public/rfc/bibxml/reference.STD.0069.xml This leads to an error: Error resolving bibliographic item. Tried methods: auto (RfcAdapter: no refs found): not found (Item not found), fallback (): not indexed A better error message would be: That item is not available here. If you are looking for STDs, instead try "bibxml9" instead of "bibxml"

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