[check_config] refactor: improve error handling (#3927)

- Combine file existence and permission checks with better error
messages
- Replace thrown exceptions with clean error output (no stack traces)
- Support custom module positions by changing strict validation to
warnings
- Add missing process.exit(1) after validation errors

Users now see clear, actionable error messages without stack traces, and
custom region configurations work correctly.

## example before

```bash
$ npm run start

> magicmirror@2.34.0-develop start
> node --run start:x11

[2025-10-22 17:56:06.303] [LOG]   Starting MagicMirror: v2.34.0-develop 
[2025-10-22 17:56:06.304] [LOG]   Loading config ... 
[2025-10-22 17:56:06.304] [LOG]   config template file not exists, no envsubst 
[2025-10-22 17:56:06.356] [ERROR] File not found: /home/kristjan/MagicMirror/config/config.js
No config file present! 
[2025-10-22 17:56:06.356] [ERROR] [checkconfig] Error: Error: ENOENT: no such file or directory, access '/home/kristjan/MagicMirror/config/config.js'
No permission to access config file!
    at checkConfigFile (/home/kristjan/MagicMirror/js/check_config.js:43:9)
    at Object.<anonymous> (/home/kristjan/MagicMirror/js/check_config.js:138:2)
    at Module._compile (node:internal/modules/cjs/loader:1714:14)
    at Module._extensions..js (node:internal/modules/cjs/loader:1848:10)
    at Module.load (node:internal/modules/cjs/loader:1448:32)
    at Module._load (node:internal/modules/cjs/loader:1270:12)
    at c._load (node:electron/js2c/node_init:2:17993)
    at TracingChannel.traceSync (node:diagnostics_channel:322:14)
    at wrapModuleLoad (node:internal/modules/cjs/loader:244:24)
    at Module.require (node:internal/modules/cjs/loader:1470:12)
    at require (node:internal/modules/helpers:147:16)
    at loadConfig (/home/kristjan/MagicMirror/js/app.js:126:3)
    at App.start (/home/kristjan/MagicMirror/js/app.js:291:18)
    at Object.<anonymous> (/home/kristjan/MagicMirror/js/electron.js:228:7)
    at Module._compile (node:internal/modules/cjs/loader:1714:14)
    at Module._extensions..js (node:internal/modules/cjs/loader:1848:10) 
```

## example after

```bash
$ npm run start

> magicmirror@2.34.0-develop start
> node --run start:x11

[2025-10-22 21:33:27.930] [LOG]   Starting MagicMirror: v2.34.0-develop 
[2025-10-22 21:33:27.931] [LOG]   Loading config ... 
[2025-10-22 21:33:27.931] [LOG]   config template file not exists, no envsubst 
[2025-10-22 21:33:27.985] [ERROR] [check_config] File not found: /home/kristjan/MagicMirror/config/config.js 
```

CHANGELOG.md

@@ -18,6 +18,7 @@ planned for 2026-01-01
 ### Changed
 
 - [core] refactor: replace `module-alias` dependency with internal alias resolver (#3893)
+- [check_config] refactor: improve error handling (#3927)
 - [calendar] test: remove "Recurring event per timezone" test (#3929)
 
 ### Fixed