arti_client: Move BadOnion from TorAddrError to ErrorDetail.
This variant can never be constructed when parsing a TorAddr, so it
doesn't make sense to have it in TorAddrError: the use can never see
it unless they enable error_detail
.
Not a semver break because the client feature is not yet stable.
Closes #932 (closed).
This is an alternative to !1329 (closed).