exception - How to fail OIM Role Provisioning on the basis of certain condition from event handler?

Question

I have a event handler which is working fine and gets executed on role assign and revoke.

Now, I want to raise exception and stop the execution of Role provisioning on certain condition.

Even though I am throwing exception Role is getting provisioned and revoked, and not producing any error on UI.

PS: This role provisioning and deprovisioning is done manually via OIM GUI.