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/scanner/src/batch/mod.rs

@@ -7,7 +7,10 @@ use serai_db::{DbTxn, Db};
 
 use serai_in_instructions_primitives::{MAX_BATCH_SIZE, Batch};
 
-use primitives::{EncodableG, task::ContinuallyRan};
+use primitives::{
+  EncodableG,
+  task::{DoesNotError, ContinuallyRan},
+};
 use crate::{
   db::{Returnable, ScannerGlobalDb, InInstructionData, ScanToBatchDb, BatchData, BatchToReportDb},
   index,
@@ -60,7 +63,9 @@ impl<D: Db, S: ScannerFeed> BatchTask<D, S> {
 }
 
 impl<D: Db, S: ScannerFeed> ContinuallyRan for BatchTask<D, S> {
-  fn run_iteration(&mut self) -> impl Send + Future<Output = Result<bool, String>> {
+  type Error = DoesNotError;
+
+  fn run_iteration(&mut self) -> impl Send + Future<Output = Result<bool, Self::Error>> {
     async move {
       let highest_batchable = {
         // Fetch the next to scan block