Log when CTAP2 bio_enrollment responses return None fields

[jkaiwar]

This PR adds info! logs in the error paths for a couple of BioEnrollment implementation methods, which previously returned Error::Ctap(CtapError::Other) silently under some CTAP requests to channels, where the response had None fields that were expected to be Some.

Maintainers: Are these logs desirable in the context of #14 ? How appropriate are the messages and log-levels I have chosen?

[AlfioEmanueleFresta]

Thanks @jkaiwar!

I believe warn! here would be most appropriate if this is unexpected authenticator behaviour, and causes the request to fail.

If this is more common, debug! would be appropriate. IMO, info! should be few and far between during normal operation, only reserved to high level methods, reporting results.

I would defer to @msirringhaus on the specifics on biometrics enrollment.

[msirringhaus]

I would agree that info!() is too low. If devices do not return these fields, they are in direct violation of the spec. So I would also go to warn!() (or maybe even higher).

[AlfioEmanueleFresta]

I would reserve error! to something the library genuinely can't handle, or high-level operation failures.

[jkaiwar]

Sounds good @AlfioEmanueleFresta @msirringhaus. I have changed the levels to warn.