Fix two misleading messages (closes #240)

The warning for a config file that others can read always said the
file contained S3 credentials, so a file:// config with none got a
false claim. When s3.access_key_id or s3.secret_access_key is set it
now says the file may contain them, because Load sees the values only
after smartconfig has replaced any ${...} reference, so a set
credential need not be in the file. Otherwise it says the file is
readable by others.

snapshot purge wrapped the listing error, which already starts with
"listing remote snapshots:", in that prefix a second time.
syncWithRemote now returns it unwrapped, as CleanupLocalSnapshots
does.

Model: opus-5-5

internal/vaultik/snapshot.go

@@ -1052,7 +1052,7 @@ func (v *Vaultik) syncWithRemote() error {
 	// every local snapshot record (issue #160).
 	remoteKeys, err := v.listAllRemoteSnapshotKeys()
 	if err != nil {
-		return fmt.Errorf("listing remote snapshots: %w", err)
+		return err
 	}
 
 	remoteKeySet := make(map[string]bool, len(remoteKeys))