plugin: Another new notification type, 'sendpay_failure'

(The json when sendpay successes is too different when sendpay fails, so
divide the sendpay result into two notifications: `sendpay_success` and
`sendpay_failure`)

`sendpay_failure`

A notification for topic `sendpay_failure` is sent every time a sendpay
success(with `failed` status). The json is same as the return value of
command `sendpay`/`waitsendpay` when this cammand fails.

```json
{
  "sendpay_failure": {
  "code": 204,
  "message": "failed: WIRE_UNKNOWN_NEXT_PEER (reply from remote)",
  "data": {
    "id": 2,
    "payment_hash": "9036e3bdbd2515f1e653cb9f22f8e4c49b73aa2c36e937c926f43e33b8db8851",
    "destination": "035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d",
    "msatoshi": 100000000,
    "amount_msat": "100000000msat",
    "msatoshi_sent": 100001001,
    "amount_sent_msat": "100001001msat",
    "created_at": 1561395134,
    "status": "failed",
    "erring_index": 1,
    "failcode": 16394,
    "failcodename": "WIRE_UNKNOWN_NEXT_PEER",
    "erring_node": "022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59",
    "erring_channel": "103x2x1",
    "erring_direction": 0
    }
  }
}
```
`sendpay` doesn't wait for the result of sendpay and `waitsendpay`
returns the result of sendpay in specified time or timeout, but
`sendpay_failure` will always return the result anytime when sendpay
fails if is was subscribed.

lightningd/notification.c

@@ -251,3 +251,52 @@ void notify_sendpay_success(struct lightningd *ld,
 	jsonrpc_notification_end(n);
 	plugins_notify(ld->plugins, take(n));
 }
+
+static void sendpay_failure_notification_serialize(struct json_stream *stream,
+						   const struct wallet_payment *payment,
+						   int pay_errcode,
+						   const u8 *onionreply,
+						   const struct routing_failure *fail,
+						   char *errmsg)
+{
+	json_object_start(stream, "sendpay_failure");
+
+	/* In line with the format of json error returned
+	 * by sendpay_fail(). */
+	json_add_member(stream, "code", false, "%d", pay_errcode);
+	json_add_string(stream, "message", errmsg);
+
+	json_object_start(stream, "data");
+	json_sendpay_fail_fields(stream,
+				 payment,
+				 pay_errcode,
+				 onionreply,
+				 fail);
+
+	json_object_end(stream); /* .data */
+	json_object_end(stream); /* .sendpay_failure */
+}
+
+REGISTER_NOTIFICATION(sendpay_failure,
+		      sendpay_failure_notification_serialize);
+
+void notify_sendpay_failure(struct lightningd *ld,
+			    const struct wallet_payment *payment,
+			    int pay_errcode,
+			    const u8 *onionreply,
+			    const struct routing_failure *fail,
+			    char *errmsg)
+{
+	void (*serialize)(struct json_stream *,
+			  const struct wallet_payment *,
+			  int,
+			  const u8 *,
+			  const struct routing_failure *,
+			  char *) = sendpay_failure_notification_gen.serialize;
+
+	struct jsonrpc_notification *n =
+	    jsonrpc_notification_start(NULL, "sendpay_failure");
+	serialize(n->stream, payment, pay_errcode, onionreply, fail, errmsg);
+	jsonrpc_notification_end(n);
+	plugins_notify(ld->plugins, take(n));
+}

lightningd/notification.h

@@ -58,4 +58,11 @@ void notify_forward_event(struct lightningd *ld,
 void notify_sendpay_success(struct lightningd *ld,
 			    const struct wallet_payment *payment);
 
+void notify_sendpay_failure(struct lightningd *ld,
+			    const struct wallet_payment *payment,
+			    int pay_errcode,
+			    const u8 *onionreply,
+			    const struct routing_failure *fail,
+			    char *errmsg);
+
 #endif /* LIGHTNING_LIGHTNINGD_NOTIFICATION_H */