gpgv: show always webportal error on NODATA

gpgv: show always webportal error on NODATA

gpg doesn't give use a UID on NODATA, which we were "expecting" (but not
using for anything), but just an error number. Instead of collecting
these as badsigners which will trigger a "invald signature" error with
remarks like "NODATA 1" we instead adapt a message similar to the NODATA
error of a clearsigned file (which is actually not reached anymore as we
split them up, which fails with a NOSPLIT error, which uses the same
general error message).

In other words: Not a security relevant change, just a user experience
improvement as we now point them to the most likely cause of the
problem instead of saying "invalid signature" which would point them in
the direction of the archive being broken (for everyone) instead.

Closes: 823746