Wrong alert returned when client doesn't provide "signature_algorithms"

Draft of TLS 1.3 says in 4.2.3:

```
If a server is authenticating via a
certificate and the client has not sent a "signature_algorithms"
extension, then the server MUST abort the handshake with a
"missing_extension" alert (see Section 9.2).
```

Currently code doesn't send this code as "missing_extension" alert is not even on a list of alerts

Trace the server-side TLS 1.3 handshake path used when authenticating with a certificate, then inspect how alert codes are defined and selected. Compare the behavior with the quoted TLS 1.3 Section 4.2.3 requirement; done means a client omitting signature_algorithms receives a missing_extension alert.

Written by the indexing model from the issue text.