Windows: Display error code in default case of windows_transfer_callback
Core: Don't wait for completion if cancel_transfer failed

* See http://libusbx.1081486.n5.nabble.com/Libusbx-devel-libusb-interrupt-transfer-does-not-return-in-case-of-error-td626.html
* Closes #76
Core: Fix warning implicit conversion changes signedness

* Closes #70