handle error OpenDeviceWithVidPid and add GetStringDescriptor

usb/device.go

@@ -203,3 +203,19 @@ found:
 
 	return end, nil
 }
+
+func (d *Device) GetStringDescriptor(desc_index int) (string, error) {
+	goBuffer := make([]byte, 200)
+	errno := C.libusb_get_string_descriptor_ascii(
+		d.handle,
+		C.uint8_t(desc_index),
+		(*C.uchar)(unsafe.Pointer(&goBuffer[0])),
+		200)
+
+	if errno < 0 {
+		return "", fmt.Errorf("usb: getstr: %s", usbError(errno))
+	}
+	stringDescriptor := string(goBuffer[:errno])
+
+	return stringDescriptor, nil
+}

usb/usb.go

@@ -112,8 +112,17 @@ func (c *Context) ListDevices(each func(desc *Descriptor) bool) ([]*Device, erro
 // OpenDeviceWithVidPid opens Device from specific VendorId and ProductId.
 // If there are any errors, it'll returns at second value.
 func (c *Context) OpenDeviceWithVidPid(vid, pid int) (*Device, error) {
+
 	handle := C.libusb_open_device_with_vid_pid(c.ctx, (C.uint16_t)(vid), (C.uint16_t)(pid))
+	if handle == nil {
+		return nil, ERROR_NOT_FOUND
+	}
+
 	dev := C.libusb_get_device(handle)
+	if dev == nil {
+		return nil, ERROR_NO_DEVICE
+	}
+
 	desc, err := newDescriptor(dev)
 
 	// return an error from nil-handle and nil-device