It's impossible to detect if keypair exists in PIV

[G1gg1L3s]

Hello! I'm doing a small CLI utility that works with Yubikey. As part of its functionality I wanted to detect whether a keypair already existing in a certain slot before generating it, to prevent users from accidentally overwriting that keypair.

But to my surprise, the implementation of metadata command can only return Error::NotSupported and Error::GenericError:

yubikey.rs/src/piv.rs

Lines 931 to 937 in b07612e

	if !response.is_success() {
	if response.status_words() == StatusWords::NotSupportedError {
	return Err(Error::NotSupported); // Requires firmware 5.2.3
	} else {
	return Err(Error::GenericError);
	}
	}

As a result, it is impossible to ditinguish between "not found" and other errors.

NIST defines a separate response code 6a 88 as "Referenced data or reference data not found" and this is exactly what Yubikey returned when I tried to execute this command on an empty slot.

I understand that returning another error type from the method may be considered as a breaking change, but it would be welcomed :)

[tony-iqlusion]

A breaking change is fine. Please open a PR.