Update error message copy

What does this MR do and why?

• Updates error message copy for clarity and brevity

Before	After
Failed to delete the role: cannot be deleted because it is already assigned to a user. Please disassociate the member role from all users before deletion.	Failed to delete role. Role is assigned to one or more group members. Remove role from all group members, then delete role.

MR acceptance checklist

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Screenshots or screen recordings

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Before	After

How to set up and validate locally

• Use SaaS with ultimate license
• Assign custom role to a group member
• Delete custom role from settings>roles & permissions