devel_switch_user_form_validate($form, &$form_state)sites/all/modules/devel/devel.module, line 706
<?php
function devel_switch_user_form_validate($form, &$form_state) {
if (!$account = user_load_by_name($form_state['values']['username'])) {
form_set_error('username', t('Username not found'));
}
}
?>