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.

Start in Site/SiteStorage.py at loadJson around line 372 and follow its call to open around line 258. Reproduce the issue by opening a site for a logged-in ID that has not posted messages, then inspect how the missing content.json is handled. Done means this condition no longer produces an unhandled exception while the site continues functioning.

索引モデルが issue の本文から書いたものです。