Unhandled exception when not connected to the Internet

Let's not assume people have Internet access all the time.

Unhandled exception when not connected to the Internet:

```
firejail ./BeingTested.AppImage zenity, kdialog, Xdialog missing. Skipping /run/firejail/appimage/.appimage-10233/AppRun.
debug: Making initial database settings...
debug: Local data storage is in "/home/travis/.config/CodeX Notes" directory
Xlib: extension "RANDR" missing on display ":99".
Checking for update
debug: [updater] 'Checking for update'
Pug interceptor registered successfully
Generated new staging user ID: 1b8a874b-6d59-5712-b41d-9e1f5b90ade5
Error: Error: net::ERR_INTERNET_DISCONNECTED
at CancellationToken.createPromise (/run/firejail/appimage/.appimage-10233/app/resources/app.asar/node_modules/builder-util-runtime/src/CancellationToken.ts:51:5)
at ElectronHttpExecutor.doApiRequest (/run/firejail/appimage/.appimage-10233/app/resources/app.asar/node_modules/builder-util-runtime/src/httpExecutor.ts:81:30)
at ElectronHttpExecutor.request (/run/firejail/appimage/.appimage-10233/app/resources/app.asar/node_modules/builder-util-runtime/src/httpExecutor.ts:73:17)
at GitHubProvider.httpRequest (/run/firejail/appimage/.appimage-10233/app/resources/app.asar/node_modules/electron-updater/src/Provider.ts:26:26)
at /run/firejail/appimage/.appimage-10233/app/resources/app.asar/node_modules/electron-updater/src/GitHubProvider.ts:38:41
at Generator.next ()
From previous event:
at GitHubProvider.getLatestVersion (/run/firejail/appimage/.appimage-10233/app/resources/app.asar/node_modules/electron-updater/out/GitHubProvider.js:167:7)
at /run/firejail/appimage/.appimage-10233/app/resources/app.asar/node_modules/electron-updater/src/AppUpdater.ts:304:25
From previous event:
at AppImageUpdater.getUpdateInfo (/run/firejail/appimage/.appimage-10233/app/resources/app.asar/node_modules/electron-updater/out/AppUpdater.js:460:7)
at /run/firejail/appimage/.appimage-10233/app/resources/app.asar/node_modules/electron-updater/src/AppUpdater.ts:308:35
at Generator.next ()
From previous event:
at AppImageUpdater.doCheckForUpdates (/run/firejail/appimage/.appimage-10233/app/resources/app.asar/node_modules/electron-updater/out/AppUpdater.js:502:7)
at /run/firejail/appimage/.appimage-10233/app/resources/app.asar/node_modules/electron-updater/src/AppUpdater.ts:279:25
at Generator.next ()
at runCallback (timers.js:789:20)
at tryOnImmediate (timers.js:751:5)
at processImmediate [as _immediateCallback] (timers.js:722:5)
From previous event:
at AppImageUpdater._checkForUpdates (/run/firejail/appimage/.appimage-10233/app/resources/app.asar/node_modules/electron-updater/out/AppUpdater.js:433:7)
at AppImageUpdater.checkForUpdates (/run/firejail/appimage/.appimage-10233/app/resources/app.asar/node_modules/electron-updater/src/AppUpdater.ts:217:35)
at App.app.on (/run/firejail/appimage/.appimage-10233/app/resources/app.asar/src/updater.js:110:17)
at emitTwo (events.js:131:20)
at App.emit (events.js:214:7)
debug: [updater] 'Error while checking for updates'
debug: [updater] 'net::ERR_INTERNET_DISCONNECTED'
```

Reference:
https://github.com/AppImage/appimage.github.io/pull/1123

从 src/updater.js 第 110 行附近的 App.app.on 处理程序开始，检查报告中所示的 electron-updater 错误路径。在没有 Internet 访问的情况下复现启动过程，然后验证应用程序仍可正常使用，并且断开连接时的更新检查不再产生未处理的异常。

由索引模型根据 Issue 内容生成。