`wallet_payment` `path_secrets` is never written to db

[ZmnSCPxj]

Nothing in wallet/wallet.c writes this column, even though it is loaded from db.

[ZmnSCPxj]

Actually, I want to bring up now, should path_secrets be saved? I think wallet_payment is only saved to db on success, but path_secrets is only relevant on failure?

[rustyrussell]

Yes. And test_payment_failed should be checking the result, but it isn't!

In particular log should contain:
"htlc %"PRIu64" failed from %ith node with code 0x%04x (%s)"

and the future should raise a Valueerror with the string WIRE_UNKNOWN_PAYMENT_HASH!

[ZmnSCPxj]

A reason I ask is because, since it is not saved in the db yet, I want to also include the route information short_channel_id of the hop also, for #638, into this path_secrets field, if possible. This will let us report the failing channel to gossipd.