internal/download: only report stale existing downloads (#34849)

## Summary

Fixes #31917.

`geth era-download` now only prints `is stale` when an existing
downloaded file fails checksum verification. Missing files are still
downloaded normally, but no longer get mislabeled as stale.

## Why

`DownloadFile` used `verifyHash` for both missing files and checksum
mismatches, then printed `is stale` for any error. This made first-time
downloads look like corrupt or outdated files.

## Validation

- `make all`
- `go run ./build/ci.go test`
- `go run ./build/ci.go lint`
- `go run ./build/ci.go check_generate`
- `go run ./build/ci.go check_baddeps`

---------

Co-authored-by: Felix Lange <fjl@twurst.com>

internal/download/download.go

@@ -22,6 +22,7 @@ import (
 	"bytes"
 	"crypto/sha256"
 	"encoding/hex"
+	"errors"
 	"fmt"
 	"io"
 	"iter"
@@ -180,12 +181,13 @@ func (db *ChecksumDB) DownloadFile(url, dstPath string) error {
 		return fmt.Errorf("no known hash for file %q", basename)
 	}
 	// Shortcut if already downloaded.
-	if verifyHash(dstPath, hash) == nil {
+	if err := verifyHash(dstPath, hash); err == nil {
 		fmt.Printf("%s is up-to-date\n", dstPath)
 		return nil
+	} else if !errors.Is(err, os.ErrNotExist) {
+		fmt.Printf("%s is stale\n", dstPath)
 	}
 
-	fmt.Printf("%s is stale\n", dstPath)
 	fmt.Printf("downloading from %s\n", url)
 	resp, err := http.Get(url)
 	if err != nil {