hid_error implementation for libusb (#698)

- hid_error is set correctly for most of the API functions (except hid_enumerate/hid_open/etc.);
- refactored iconv routines - common code for utf16 and utf8 to wchar_t implementation;
- `hid_libusb_error` to have libusb error code when possible;

Closes: #690
Fixes: #684

Co-authored-by: Matheus Afonso Martins Moreira <matheus@matheusmoreira.com>

Author: Youw