[electron-updater] Expose location of cacheDir

Use case: When installing an update is unsuccessful, it would be possible to provide user where to save the latest downloaded installer to install it manually.

So it would be nice to have the full path to the downloaded installer, or at least a location where updates are downloaded.

Workaround:

// @ts-ignore
const cacheDir: string = autoUpdater.downloadedUpdateHelper.cacheDir);

[DovydasNavickas]

The use case is quite simple: The auto updater fails (for Ubuntu in this case), because it lacks permissions or some other reason. We want to give user exactly that: “An aut-update has failed. Please try again or download here and install it manually.”

Saying that auto-updater fails only when users quickly shut down or sleep their PCs is more than optimistic, cause’ there might be gazillion of errors with permissions, network and many more.

Thus, it’d be nice to have the API to get:

• full download URL
• location where file was saved

This way we can solve the problem without digging into internal stuff by going around TypeScript with // @ts-ignore 👍

[develar]

update-downloaded event now has property downloadedFile.