/// In order to ensure we always return an error on onion decode in compliance with [BOLT
/// #4](https://github.com/lightning/bolts/blob/master/04-onion-routing.md), we have to
/// deserialize `OnionPacket`s contained in [`UpdateAddHTLC`] messages even if the ephemeral
/// public key (here) is bogus, so we hold a [`Result`] instead of a [`PublicKey`] as we'd
/// like.
/// In order to ensure we always return an error on onion decode in compliance with [BOLT
/// #4](https://github.com/lightning/bolts/blob/master/04-onion-routing.md), we have to
/// deserialize `OnionPacket`s contained in [`UpdateAddHTLC`] messages even if the ephemeral
/// public key (here) is bogus, so we hold a [`Result`] instead of a [`PublicKey`] as we'd
/// like.