Etcd auth Inaccurate logging of authentication attempts for users with CN-based auth only

Summary

### Vulnerability type Logging

### Detail etcd users who have no password can authenticate only through a client certificate. When such users try to authenticate into etcd using the Authenticate endpoint, errors are logged with insufficient information regarding why the authentication failed, and may be misleading when auditing etcd logs.

### References Find out more on this vulnerability in the [security audit report](https://github.com/etcd-io/etcd/blob/master/security/SECURITY_AUDIT.pdf)

### For more information If you have any questions or comments about this advisory: * Contact the [etcd security committee](https://github.com/etcd-io/etcd/blob/master/security/security-release-process.md#product-security-committee-psc)