[QA] Admin Tool | Error shown when re-adding existing Admin

### 📔 **Issue Description:**
When trying to add an Admin that was already added before, the Admin Tool shows a generic error message instead of informing that the user is already an Admin.
Since changes are not reflected until the scene is reloaded, this behavior is confusing and leads to assuming the action failed.

### 👣 **STR:**
1. Open Creator Hub Admin Tool.
2. Add a new Admin.
3. Without reloading the scene, try to add the same Admin again.
4. Observe the error message.

### ✔️ **Expected Result:**
A clear message should be shown indicating that the user is already added as Admin (e.g. “User already exists as Admin”).

### ❌ **Actual Result:**
A generic error is displayed with no explanation, making it seem the action failed.

### ➗ **Reproduction:**
🔁 Always – Happens when re-adding an existing Admin without reloading the scene.

#### 🖥️ **Operative system and additional Notes:**
Windows and Mac
Changes are only visible after reloading the scene, increasing confusion for the user. (we experience this scenario during MF25 with team members)