core: Detect unreadable keyrings.

* src/keylist.c (op_data_t): Add field keydb_search_err.
(keylist_status_handler): Parse STATUS_ERROR into that var.
(gpgme_op_keylist_next): Use that err instead of GPG_ERR_EOF.

With a recent GnuPG version this allows to detect a keyring with wrong
permissions (i.e. no read access). GPG_ERR_OPEN_KEYRING will be
returned instead of GPG_ERR_EOF.

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