/*
 * call-seq:
 *   easy.on_failure {|easy,code| ... }               => <old handler>
 *
 * Assign or remove the +on_failure+ handler for this Curl::Easy instance.
 * To remove a previously-supplied handler, call this method with no
 * attached block.
 *
 * The +on_failure+ handler is called when the request is finished with a
 * status of 50x
 */
static VALUE ruby_curl_easy_on_failure_set(int argc, VALUE *argv, VALUE self) {