cmd/era: fix iterator error source handling in checkAccumulator (#32698)

This change replaces wrapping a stale outer err with the iterator’s own
error after Next(), and switches the post-BlockAndReceipts() check to
use the returned err. According to internal/era iterator contract,
Error() should be consulted immediately after Next() to surface
iteration errors, while decoding errors from Block/Receipts are returned
directly. The previous code could hide the real failure (using nil or
unrelated err), leading to misleading diagnostics and missed iteration
errors.

---------

Co-authored-by: lightclient <lightclient@protonmail.com>

cmd/era/main.go

@@ -274,10 +274,10 @@ func checkAccumulator(e *era.Era) error {
 	for it.Next() {
 		// 1) next() walks the block index, so we're able to implicitly verify it.
 		if it.Error() != nil {
-			return fmt.Errorf("error reading block %d: %w", it.Number(), err)
+			return fmt.Errorf("error reading block %d: %w", it.Number(), it.Error())
 		}
 		block, receipts, err := it.BlockAndReceipts()
-		if it.Error() != nil {
+		if err != nil {
 			return fmt.Errorf("error reading block %d: %w", it.Number(), err)
 		}
 		// 2) recompute tx root and verify against header.
@@ -294,6 +294,9 @@ func checkAccumulator(e *era.Era) error {
 		td.Add(td, block.Difficulty())
 		tds = append(tds, new(big.Int).Set(td))
 	}
+	if it.Error() != nil {
+		return fmt.Errorf("error reading block %d: %w", it.Number(), it.Error())
+	}
 	// 4+5) Verify accumulator and total difficulty.
 	got, err := era.ComputeAccumulator(hashes, tds)
 	if err != nil {