util: wrap unknown-device erroring in just one function

The way to error out about an unknown device was being
triplicated in the code.

This commit shouldn't bring any behaviour change, as it's
just refactoring.
diff --git a/src/libusb1-glue.c b/src/libusb1-glue.c
index 2a6041f..4728aac 100644
--- a/src/libusb1-glue.c
+++ b/src/libusb1-glue.c
@@ -693,18 +693,7 @@
       }
     }
     if (!device_known) {
-      // This device is unknown to the developers
-      LIBMTP_ERROR("Device %d (VID=%04x and PID=%04x) is UNKNOWN.\n",
-	      i,
-	      desc.idVendor,
-	      desc.idProduct);
-      LIBMTP_ERROR("Please report this VID/PID and the device model to the "
-	      "libmtp development team\n");
-      /*
-       * Trying to get iManufacturer or iProduct from the device at this
-       * point would require opening a device handle, that we don't want
-       * to do right now. (Takes time for no good enough reason.)
-       */
+      device_unknown(i, desc.idVendor, desc.idProduct);
     }
     // Save the location on the bus
     retdevs[i].bus_location = libusb_get_bus_number (dev->device);