Improve error message on attestation verification failure

[woodruffw]

Filing this for myself to fix/improve 🙂

If a user uploads a funky attestation (i.e. one that doesn't roughly match our expectations for a Sigstore issued machine identity cert), the upload endpoint produces a pretty opaque error message:

WARNING  Error during upload. Retry with the --verbose option for more details. 
ERROR    HTTPError: 400 Bad Request from https://upload.pypi.org/legacy/        
         Invalid attestations supplied during upload: Unknown error while trying
         to verify included attestations: No                                    
         <ObjectIdentifier(oid=1.3.6.1.4.1.57264.1.14, name=Unknown OID)>       
         extension was found     

(This particular error case was a bug, now fixed with #17913. However, in the general case this still produces a non-ideal error.)