Improve Template/ISO Failure Status and Error Reporting

### The required feature described as a wish

**Description:**
In the ISO/Templates flow, error responses currently return detailed error messages directly in the status field. This leads to inconsistencies and complicates error handling.

**Proposal:**
Introduce two improvements to the API responses for Templates and ISOs:

1) Standardize the `status` Field for Failures
When any terminal error occurs during the processing of a template or ISO, the `status` field should consistently return `"Failed"`. This provides a clear and unambiguous final state for parsing.

2) Add a Detailed Error Message Field
A new field, named `error_details`, should be added to the Template/ISO API response object.
This field will contain a detailed error string explaining the reason for the failure.

**Example of the response**
Failed download status of the ISO/Template:
```
{
"id": "a1b2c3d4-xxxx-yyyy-zzzz-e5f6g7h8i9j0",
"name": "my-failed-template",
"status": "Failed",
"error_details": "HTTP Server returned 404 (expected 200 OK)",
....
}
```
Successful download status of the ISO/Template:
```
{
"id": "b1b2c3d4-xxxx-yyyy-zzzz-e5f6g7h8i9j1",
"name": "my-template",
"status": "Download Complete",
"error_details": null,
....
}
```
**Benefits:**
- Consistent error structure across ISO/Templates;
- Easier parsing, logging, and UI display of error messages;
- Avoids issues caused by leading/trailing spaces or unexpected values in status.

Tasks:
1) Update ISO/Templates logic to return "Failed" in the `status` field.
2) Return detailed error messages in the `error_details` field.

Commencez par suivre le traitement des réponses de l’API ISO/Templates et les chemins d’erreur du terminal décrits dans l’issue. Vérifiez chaque réponse d’échec ainsi que la réponse de téléchargement réussie, puis vérifiez que les échecs utilisent "Failed" avec le message détaillé dans error_details et que les réponses réussies utilisent une valeur null pour error_details.

Rédigé par le modèle d'indexation à partir du texte de l'issue.