renepay: Crash on unparseable sendpay response

[cdecker]

I saw this crash in CI (run 43856688685):

lightningd-1 2025-06-11T02:44:20.523Z DEBUG   plugin-cln-renepay: sendpay_failure notification: {"sendpay_failure":{"code":202,"message":"Malformed error reply","data":{"created_index":2,"id":2,"payment_hash":"e0079bd8c4827073e6ced50e32400a2c401dd53ebf579e04c218f4ccf718bcf8","groupid":1,"partid":1,"destination":"0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518","amount_msat":10000,"amount_sent_msat":10000,"created_at":1749609860,"status":"pending","bolt11":"lnbcrt100n1p5y3mvysp5d8364lnpvkaky9g2nwludmzl27v92p5jav3ece7n5e44e83wv5vqpp5uqrehkxysfc88ekw658rysq293qpm4f7hateupxzrr6veacchnuqdqgw3jhxapjxqyjw5qcqp99qxpqysgqafzxempvyne872vrk005nadpnmua2cyqf7llrwfqsu9k9ua9rxk43umzqndk08p0qy44974zcmxn4rmy7epngpr540q7t6c7qgy4hlspcjlxxc","onionreply":"584f1d984a6a0d0adee0e128ca49e3540d07162f5440edc07f6d56e5cf040118000e400f00000000000027100000006c00f20000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"}}}
lightningd-1 2025-06-11T02:44:20.523Z **BROKEN** plugin-cln-renepay: Unable to parse sendpay_failure: {"code":202,"message":"Malformed error reply","data":{"created_index":2,"id":2,"payment_hash":"e0079bd8c4827073e6ced50e32400a2c401dd53ebf579e04c218f4ccf718bcf8","groupid":1,"partid":1,"destination":"0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518","amount_msat":10000,"amount_sent_msat":10000,"created_at":1749609860,"status":"pending","bolt11":"lnbcrt100n1p5y3mvysp5d8364lnpvkaky9g2nwludmzl27v92p5jav3ece7n5e44e83wv5vqpp5uqrehkxysfc88ekw658rysq293qpm4f7hateupxzrr6veacchnuqdqgw3jhxapjxqyjw5qcqp99qxpqysgqafzxempvyne872vrk005nadpnmua2cyqf7llrwfqsu9k9ua9rxk43umzqndk08p0qy44974zcmxn4rmy7epngpr540q7t6c7qgy4hlspcjlxxc","onionreply":"584f1d984a6a0d0adee0e128ca49e3540d07162f5440edc07f6d56e5cf040118000e400f00000000000027100000006c00f20000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"}}

We should likely not just crash the renepay plugin, instead clean up and report the error, and keep moving.

[Lagrang3]

It would not crash on any particular payment outcome.
This crash means that renepay was unable to parse the sendpay_failure notification sent from lightningd,
that would be a bug that should not be ignored IMO.

It is weird that I am not able to reproduce this locally.