gpg: Don't let gpg return failure on an invalid packet in a keyblock.

* g10/keydb.c (parse_keyblock_image): Use log_info instead of
log_error for skipped packets.
* g10/keyring.c (keyring_get_keyblock): Ditto.

log_info should be sufficient and makes this more robust. Some
tools (e.g. Enigmail) are too picky on return codes from gpg.

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