Cohorts already require unique idnumbers which is good. However, you can create x amount of cohorts each with NO idnumber. This caused a problem when running cron that said:

Running cron for enrol_cohort...
!!! Coding error detected, it must be fixed by a programmer: User ID does not exist or is deleted! !!!

Here is the easy fix:

open /cohort/edit_form.php
look for
public function definition() {

look for
$mform->addElement('text', 'idnumber', get_string('idnumber', 'cohort'), 'maxlength="254" size="50"');
inside that function and add

$mform->addRule('idnumber', get_string('required'), 'required', null, 'client');

below it.

This fixed it for us adding and editing cohorts but I have no idea if there is anywhere else it should be modified OR if the line I have added will break anything else...have removed it for now till you guys decide to use this fix or not...