gpg: Show debugging info if a sig with an unsupported sig class is used.
* g10/sig-check.c (check_key_signature2): If SIG->CLASS is unsupported, show some debugging information. Don't use BUG to fail. Just return GPG_ERR_BAD_SIGNATURE.
- Signed-off-by: Neal H. Walfield <neal@g10code.com>