SSL: ngx_ssl_set_client_hello_callback() error handling.

SSL: ngx_ssl_set_client_hello_callback() error handling.

The function interface is changed to follow a common approach
to other functions used to setup SSL_CTX, with an exception of
"ngx_conf_t *cf" since it is not bound to nginx configuration.

This is required to report and propagate SSL_CTX_set_ex_data()
errors, as reminded by Coverity (CID 1668589).