Missing error message for untestable verification rules

[ccanash]

In a test plan, if a user marks a command as untestable, they have to set at least one negative side effect to severe.
Currently, the error message is not being displayed

This was reported in w3c/aria-at#1278

[howard-e]

After some investigation into this, seems the fix should be applied in aria-at. Submitted w3c/aria-at#1295 which should address it.