Confusing error message

[kbabioch]

When running ykpamcfg without a YubiKey attached, I get the following error messages:

$ ykpamcfg -v -A123
Unknown action '123'
USB error: Success (no error)

This error message should be improved, since it is somewhat confusing (USB error vs. success / no error).

The code in question is this:

yubico-pam/ykpamcfg.c

Lines 81 to 82 in fa3833b

	fprintf(stderr, "USB error: %s\n",
	yk_usb_strerror());