more verbose error message if opening the usb device fails

src/librtlsdr.c

@@ -1330,7 +1330,10 @@ int rtlsdr_open(rtlsdr_dev_t **out_dev, uint32_t index)
 	r = libusb_open(device, &dev->devh);
 	if (r < 0) {
 		libusb_free_device_list(list, 1);
-		fprintf(stderr, "usb_open error %d\n", r);
+		fprintf(stderr, "usb_open error %s\n", libusb_error_name(r));
+		if(r == LIBUSB_ERROR_ACCESS)
+			fprintf(stderr, "Please fix the device permissions, e.g. "
+			"by installing the udev rules file rtl-sdr.rules\n");
 		goto err;
 	}
 
@@ -1338,7 +1341,8 @@ int rtlsdr_open(rtlsdr_dev_t **out_dev, uint32_t index)
 
 	r = libusb_claim_interface(dev->devh, 0);
 	if (r < 0) {
-		fprintf(stderr, "usb_claim_interface error %d\n", r);
+		fprintf(stderr, "usb_claim_interface error %s\n",
+			libusb_error_name(r));
 		goto err;
 	}