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?

Start by tracing the reported call path through Bigfile/BigfilePlugin.py at line 356 and core/src/Site/SiteStorage.py at line 257, using the debug.log details and the missing .piecemap.msgpack path. Determine whether the missing file is expected for these sites and define the desired handling for the repeated FileNotFoundError.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.