dtls_server: improve handling of unexpected messages during the handshake

[gilles-peskine-arm]

The dtls_server sample program can detect MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE during the second or more handshake. This error can happen if the client sends a close_notify alert after the server has decided to close the connection, or if there's a duplicated packet from the first connection.

Since #9655, dtls_server reports MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE as not-really-an-error if it happens during a handshake. This downplays the error, which can definitely also be a legitimate error in the real world, if a non-DTLS or misbehaving client tries to connect to the server.

The goal of this issue is to improve the behavior or guidance in dtls_server, to allow users to make an informed decision on how to treat this error. This likely means being more discriminatory than “mbedtls_ssl_handshake returned MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE”.