agent: Appropriate error code for importing key with no passwd.

* agent/cvt-openpgp.c (convert_from_openpgp_main): Return
GPG_ERR_BAD_SECKEY.

When non-protected case, error at gcry_pk_testkey results
GPG_ERR_BAD_PASSPHRASE.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>