If we failed after we register (e.g. channeld not available), we don't
mark it failed. We shouldn't register until we've definitely created
the htlc.
Changelog-Fixed: `xpay` would sometimes leave payment parts status `pending` in failure cases (as seen in listpays or listsendpays).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Fixes: https://github.com/ElementsProject/lightning/issues/8629
ac60568398