MINOR: ssl: Add new ssl_fc_hsk_err sample fetch

MINOR: ssl: Add new ssl_fc_hsk_err sample fetch

This new sample fetch along the ssl_fc_hsk_err_str fetch contain the
last SSL error of the error stack that occurred during the SSL
handshake (from the frontend's perspective). The errors happening during
the client's certificate verification will still be given by the
ssl_c_err and ssl_c_ca_err fetches. This new fetch will only hold errors
retrieved by the OpenSSL ERR_get_error function.