Horizontal form and has-error on a single field

Created by: odan

I have two input fields in a form-horizontal > form-group. The first field is required and the second field is optional. If the first field is empty I only want to show an 'has-error' for the first input field and not for the whole form-group.

Here is an example: http://jsfiddle.net/6968cyqr/

How can i set an 'has-error' for a single input field?