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:?}"))`.

coordinator/src/substrate.rs

@@ -32,7 +32,8 @@ pub(crate) struct SubstrateTask<P: P2p> {
 }
 
 impl<P: P2p> ContinuallyRan for SubstrateTask<P> {
-  fn run_iteration(&mut self) -> impl Send + Future<Output = Result<bool, String>> {
+  type Error = String; // TODO
+  fn run_iteration(&mut self) -> impl Send + Future<Output = Result<bool, Self::Error>> {
     async move {
       let mut made_progress = false;