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.

首先，追蹤 issue 中描述的 ISO/Templates API 回應處理和終端錯誤路徑。檢查每個失敗回應和成功下載回應，然後驗證失敗時使用帶有 error_details 中詳細訊息的 "Failed"，成功回應則對 error_details 使用 null 值。

由索引模型根據 Issue 內容生成。