[#1502] Use zap.Error for logging errors

Use `zap.Error` instead of `zap.String` for logging errors: change all expressions like
`zap.String("error", err.Error())` or `zap.String("err", err.Error())` to `zap.Error(err)`.
Leave similar expressions with other messages unchanged, for example,
`zap.String("last_error", lastErr.Error())` or `zap.String("reason", ctx.Err().Error())`.

This change was made by applying the following patch:
```diff
@@
var err expression
@@
-zap.String("error", err.Error())
+zap.Error(err)

@@
var err expression
@@
-zap.String("err", err.Error())
+zap.Error(err)
```

Signed-off-by: Aleksey Savchuk <a.savchuk@yadro.com>

pkg/services/object/util/log.go

@@ -14,7 +14,7 @@ func LogServiceError(ctx context.Context, l *logger.Logger, req string, node net
 	l.Error(ctx, logs.UtilObjectServiceError,
 		zap.String("node", network.StringifyGroup(node)),
 		zap.String("request", req),
-		zap.String("error", err.Error()),
+		zap.Error(err),
 	)
 }
 
@@ -22,6 +22,6 @@ func LogServiceError(ctx context.Context, l *logger.Logger, req string, node net
 func LogWorkerPoolError(ctx context.Context, l *logger.Logger, req string, err error) {
 	l.Error(ctx, logs.UtilCouldNotPushTaskToWorkerPool,
 		zap.String("request", req),
-		zap.String("error", err.Error()),
+		zap.Error(err),
 	)
 }