KeyError - sidebarGetHtmlTag - SidebarPlugin.py

### Step 1: Please describe your environment

* ZeroNet version: 3853
* Operating system: W10
* Web browser: Firefox
* Tor status: available
* Opened port: no
* Special configuration: multiuser_local

### Step 2: Describe the problem:

Hello, i visited a hub zite several times in last days and its zeronet menu (corner button) shows nothing in the menu, only black and debug.log shows this site relevant entry:

> ERROR Site:abc..def WebSocket handleRequest error: KeyError: u'folder/subfolder /content.json' in UiWebsocket.py line 83 > UiWebsocket.py line 267 > SidebarPlugin.py line 531 > SidebarPlugin.py line 196 > ContentManager.py line 322 > ContentManager.py line 322 > ContentManager.py line 320 > ContentDbDict.py line 61 > ContentDbDict.py line 34
{"cmd":"sidebarGetHtmlTag","params":{},"id":1000002}

(subfolder name contains ":" and there is space at the end. I checked and that folder does not exist on my end.

When i deleted site and opened again, menu started opening. If anyone interested here how i deleted the site while preserving all optional files i downloaded:

> 1. I backed up "folder"
> 2. deleted site
> 3. visited site
> 4. copied back "folder" (and skipped files that existed on newly downloaded site)
> 5. on ZeroHello i "Check files" of that site
> 6. site menu now opens

Start with the sidebarGetHtmlTag request in SidebarPlugin.py, then follow the traceback through UiWebsocket.py and ContentManager.py to the ContentDbDict.py lookup. Reproduce the malformed folder/subfolder content.json case and determine how the missing key reaches the sidebar. Done means the sidebar menu opens without this KeyError for the reported site state.

Written by the indexing model from the issue text.