Have the Eventuality task drop outputs which aren't ours and aren't worth it to aggregate

We could drop these entirely, yet there's some degree of utility to be able to
add coins to Serai in this manner.

processor/scanner/src/scan/mod.rs

@@ -226,7 +226,10 @@ impl<D: Db, S: ScannerFeed> ContinuallyRan for ScanTask<D, S> {
               costs_to_aggregate.entry(balance.coin)
             {
               e.insert(self.feed.cost_to_aggregate(balance.coin, &block).await.map_err(|e| {
-                format!("couldn't fetch cost to aggregate {:?} at {b}: {e:?}", balance.coin)
+                format!(
+                  "ScanTask couldn't fetch cost to aggregate {:?} at {b}: {e:?}",
+                  balance.coin
+                )
               })?);
             }
             let cost_to_aggregate = costs_to_aggregate[&balance.coin];