Draft: Update error message for custom role deletion

What does this MR do and why?

• Updates error message copy when deleting a custom role for consistency and brevity.

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Before	After

How to set up and validate locally

• Ensure you have an ultimate license
• From group member page, assign custom role to a group member
  • If there are no custom roles, create a custom role for a group
• Go to settings>roles & permissions (Admin area when in Self-Managed) and delete the custom role