Add a proper error type to ContinuallyRan

This isn't necessary. Because we just log the error, we never match off of it,
we don't need any structure beyond String (or now Debug, which still gives us
a way to print the error). This is for the ergonomics of not having to
constantly write `.map_err(|e| format!("{e:?}"))`.

processor/bin/src/coordinator.rs

@@ -95,6 +95,7 @@ impl Coordinator {
           message_queue.ack(Service::Coordinator, msg.id).await;
 
           // Fire that there's a new message
+          // This assumes the success path, not the just-rebooted-path
           received_message_send
             .send(())
             .expect("failed to tell the Coordinator there's a new message");