Revert "Use libusb_strerror to translate erro codes to strings."

This reverts commit 592e893a01fc4517dd27018dd71c8520c521135c.

It causes regressions on systems using old versions of libusb that
don't implement libusb_strerror.

closes #57 #68.

Conflicts:
	usb/backend/libusb1.py