Misleading authentication error message for non-existent user

When deploying a cluster with basic enabled authentication, authentication a non-existent user returns an authorization error instead of authentication error. If would be useful to return a correct message to the end-user:
- If user exists, but does not have permissions to execute a statement - return authorization error.
- If user does not exist - return an authentication error.

Current implementation always returns 'Authorization Failed' whether user exists or existing user does not have required permissions.