A Proper error message has to through while roles deletion, If there is any user associated with roles.
Steps to reproduce:
Login to zanata as admin.
Go to admin section.
Click on Manage Roles.
Create two roles role1 and role2.
Create new user and assign him role1.
Now try to delete both roles.
Observe the role1 deletion.
abc user is associated with this, are you sure you wants to delete this role. please change the role of user or delete it.
attached screen captured.