No error message when configuration file does not exist - param config_file

### Step 1: Please describe your environment

* ZeroNet version: 0.7.1 r4214
* Operating system: Fedora 30 x64
* Web browser: Chromium
* Tor status: available
* Opened port: no

### Step 2: Describe the problem:

I tried to load configuration file (`--config_file` param) but I made typo in the file name. Instead of see error message ZeroNet client created new configuration file.

#### Steps to reproduce:

1. run `python3 zeronet.py --config_file file_does_not_exist.conf`

#### Observed Results:

ZeroNet client created new file `file_does_not_exist.conf` with content

```
[global]
fileserver_port = 24124
```

#### Expected Results:

I should see error message and ZeroNet should exit
```
Cannot load configuration file file_does_not_exist.conf. Exiting ...
```

Start with the --config_file argument handling exercised by `python3 zeronet.py --config_file file_does_not_exist.conf`. Reproduce the missing-file case and verify that it reports the expected error, exits, and does not create a new configuration file.

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