How to fail the task sequence if an MDT variable is not populated?

We are deploying Windows 7 using ConfigMgr 2012 SP1 CU3 integrated with MDT 2012 Update 1 using the MDT database locations table to populate the MachineObjectOU MDT variable.

Currently the "Apply Network Settings" task will read the MachineObjectOU variable and add the computer account to the OU returned by the MDT database query.  If no value is returned it will not domain join the computer, instead it continues the task sequence and configures the machine as a workgroup machine.

I would like the task sequence to fail if the MachineObjectOU variable is not populated by the MDT database query instead of continuing as a workgroup build.  What is the easiest way to do this?

Thanks!

FP