Introduce Fork struct to block_producer

It's a pretty crappy solution, IMO. It shouldn't really belong in
"duties" but this gets the job done for now.

validator_client/src/block_producer_service/mod.rs

@@ -50,6 +50,9 @@ impl<T: SlotClock, U: BeaconNode, V: DutiesReader, W: Signer> BlockProducerServi
                 Ok(BlockProducerPollOutcome::ValidatorIsUnknown(slot)) => {
                     error!(self.log, "The Beacon Node does not recognise the validator"; "slot" => slot)
                 }
+                Ok(BlockProducerPollOutcome::UnableToGetFork(slot)) => {
+                    error!(self.log, "Unable to get a `Fork` struct to generate signature domains"; "slot" => slot)
+                }
             };
 
             std::thread::sleep(Duration::from_millis(self.poll_interval_millis));