device: return error from Up() and Down()

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

main_windows.go

@@ -48,7 +48,11 @@ func main() {
 	}
 
 	device := device.NewDevice(tun, logger)
-	device.Up()
+	err = device.Up()
+	if err != nil {
+		logger.Errorf("Failed to bring up device: %v", err)
+		os.Exit(ExitSetupFailed)
+	}
 	logger.Verbosef("Device started")
 
 	uapi, err := ipc.UAPIListen(interfaceName)