Horizon: Silent error "Content too large" & "Invalid size" of the data.json

### Step 1: Please describe your environment

* ZeroNet version: 3597
* Operating system: Linux
* Web browser: FF
* Tor status: always
* Opened port: no

### Step 2: Describe the problem:

In [this comment](https://github.com/blurHY/Horizon/issues/12#issuecomment-423489989) i was invited to submit this issue. [Horizon](http://127.0.0.1:43110/1CjMsvhJ2JsV4B5qo3FDHnF3mvRCcHuxBn/) site is downloaded, but it had problem with "Content too large" & "Invalid size" of the data.json but this issue was silent (in debug.log).

Reproduce the Horizon download described for ZeroNet 3597 on Linux with Tor, focusing on the data.json "Content too large" and "Invalid size" errors. Start by inspecting debug.log and the download path; done means the failure is no longer silent and the relevant error is reported clearly.

索引モデルが issue の本文から書いたものです。