net: sockets: tls: Use better error code

Mbed TLS 3.0 removes the definition for MBED_ERR_SSL_PEER_VERIFY_FAILED,
since non of its code ever returns that value.  Since there isn't really
a perfect response, instead return a somewhat generic response
indicating this was unexpected.

Signed-off-by: David Brown <david.brown@linaro.org>