Unhandled exception: FileNotFoundError

* ZeroNet version: rev 4471
* Operating system: Linux

When I open a site, where I have logged in with my ID, but haven't posted any messages yet, the following exception occurs:

```
[11:32:23] - Unhandled exception: FileNotFoundError: [Errno 2] No such file or directory: './data/1Vp5LH4wegCaqeB72yMw2jgNdVm4aR7ET/data/users/1GooUE19488nDwG3TdkM8seYAHct4gjkq4/content.json' in gevent/threadpool.py line 137 > SiteStorage.py line 372 > 258
Traceback (most recent call last):
File "/usr/lib/python3.8/site-packages/gevent/threadpool.py", line 137, in __run_task
thread_result.set(func(*args, **kwargs))
File "/root/src/Site/SiteStorage.py", line 372, in loadJson
with self.open(inner_path, "r", encoding="utf8") as file:
File "/root/src/Site/SiteStorage.py", line 258, in open
return open(file_path, mode, **kwargs)
FileNotFoundError: [Errno 2] No such file or directory: './data/1Vp5LH4wegCaqeB72yMw2jgNdVm4aR7ET/data/users/1GooUE19488nDwG3TdkM8seYAHct4gjkq4/content.json'
```

Doesn't seem to have any negative impact on the functioning of the site or ZeroNet. However I guess the missing file condition can be handled in more graceful way.