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

src/updater.js の 110 行目付近にある App.app.on ハンドラーから始め、レポートに示されている electron-updater のエラーパスを確認します。インターネット接続なしで起動を再現し、その後もアプリケーションが使用可能な状態を維持し、接続なしでの更新チェックによって未処理の例外が発生しなくなったことを検証します。

索引モデルが issue の本文から書いたものです。