(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.
82e8db4ba4