Fixed error going undetected if device cant be queried

```verbose_device_search()``` does not check the return value of ```rtlsdr_get_device_usb_strings()``` and outputs garbage to stderr in that case:

```
Found 1 device(s):
  0:  `.�K, ��o�K, SN: ����

Using device 0: Terratec Cinergy T Stick RC (Rev.3)
usb_open error -3
Please fix the device permissions, e.g. by installing the udev rules file rtl-sdr.rules
Failed to open rtlsdr device #0.
```

After the fix:

```
Found 1 device(s):
  0: Failed to get device description, error -3.

Using device 0: Terratec Cinergy T Stick RC (Rev.3)
usb_open error -3
Please fix the device permissions, e.g. by installing the udev rules file rtl-sdr.rules
Failed to open rtlsdr device #0.
```