Client: Warn when connecting via untrusted certificate

From Part B:

A SiLA Client MUST NOT accept untrusted certificates for TLS handshake implicitly. A SiLA Client MAY accept untrusted certificates for TLS handshake explicitly (e.g. by notifying a user or unsuring the trust in another appropriate way).

Thus, when using the CA communicated via Zeroconf TXT records, a warning must be issued. In all other cases, the user explictly provided the certificate to the client.