FileNotFoundError: [Errno 2] No such file or directory - BigfilePlugin.py

### Step 1: Please describe your environment

* ZeroNet version: 4397
* Operating system: W10
* Web browser: FF beta
* Tor status: available
* Opened port: no

### Step 2: Describe the problem:

> ERROR - Unhandled exception: FileNotFoundError: [Errno 2] No such file or directory: 'U:/ZeroNet-win-dist-win64/data/1N77kxgd29cR8w9xt481JDJEtYdX1hCqAR/data/users/1GrTv5QgmApo9jMgoWFFq1RwUpQWqtbhnQ/Arkadij-i-Boris-Strugackie-Hishchnye-veshchi-veka.mp3.piecemap.msgpack' in greenlet.py line 766 > core/src/Worker/Worker.py line 208 > ... > Bigfile/BigfilePlugin.py line 356 > 343 > 338 > core/src/Site/SiteStorage.py line 257

I see the error "FileNotFoundError: [Errno 2]" 6 times in debug.log for total 2 sites
Maybe this is not a big problem and can be ignored?

首先，使用 debug.log 中的详细信息和缺失的 .piecemap.msgpack 路径，跟踪经过 Bigfile/BigfilePlugin.py 第 356 行和 core/src/Site/SiteStorage.py 第 257 行的报告调用路径。确定对于这些站点而言该文件缺失是否属于预期情况，并定义对重复出现的 FileNotFoundError 所需的处理方式。

由索引模型根据 Issue 内容生成。