Set to validate true

test.php

        $form['results_area']['settings']['os_browser']['os'] = array(
            '#type' => 'select',
            '#options' => array('Windows 2012' => 'Windows 2012'),
            '#default_value' => 'Windows 2012',
            '#validated' => TRUE,
            '#description' => t('What OS and Browser should be used for this test')
        );

        $form['results_area']['settings']['os_browser']['browser'] = array(
            '#type' => 'select',
            '#options' => array('chrome|31' => 'Google Chrome - 31'),
            '#empty_value' => '--choose OS first--',
            '#default_value' => 'chrome|31',
            '#validated' => TRUE,
            '#description' => t('What Browser do you want to use')
        );