core: For a failed verification return the sig's fingerprint.

* src/verify.c (parse_new_sig): Parse the new ERRSIG fpr.

This works only when the signatures features an ISSUER_FPR sub-packet
and with GnuPG >= 2.2.7. If that is not the case the keyid is kept in
the FPR field.

• Signed-off-by: Werner Koch <wk@gnupg.org>