tor-proto: better errors when handshake fails due to untimely certs

We now check the handshake certificates unconditionally, and only
report them as _expired_ as a last resort.

(Rationale: if somebody is presenting the wrong identity from a year
ago, it is more interesting that they are presenting the wrong ID
than it is that they are doing so with an expired cert.

We also now report a different error if the certificate is expired,
but its expiration is within the range of reported clock skew.

(Rationale: it's helpful to distinguish this case, so that we can
blame the failure on possible clock skew rather than definitely
attributing it to a misbehaving relay.)

Part of #405.