Function handle_undeliverable_message 

pub fn handle_undeliverable_message<A: Actor>(
    _cx: &Instance<A>,
    reason: UndeliverableReason,
    undeliverable: Undeliverable<MessageEnvelope>,
) -> Result<(), Error>

Default implementation of Actor::handle_undeliverable_message. Defined as a free function so that Actor implementations that override Actor::handle_undeliverable_message can fallback to this default.