 * Fix crash when canceling a download (Tobi823/ffupdater#77)
 * Ignore error and retry background update (with exponential increasing waiting time) for CancellationException, GithubRateLimitExceededException and UnknownHostException (Tobi823/ffupdater#74)
 * Improve error handling
 * Fix download error for Focus/Klar by switching to the GitHub API (Tobiwan/ffupdater#81)