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

Start with src/updater.js around the App.app.on handler at line 110 and review the electron-updater error path shown in the report. Reproduce startup without Internet access, then verify that the application remains usable and the disconnected update check no longer produces an unhandled exception.

Written by the indexing model from the issue text.