[BUG] Index Rules API doesn't return the details of the errors while creating sigma rules

[goyamegh]

What is the bug?

How can one reproduce the bug?
Steps to reproduce the behavior:

1. Go to 'Detection rules page'
2. Click on 'Create rules button'
3. Choose the YAML editor format and enter the below input:

{
    "title": "Moriya Rootkit",
    "description": "Detects the use of Moriya rootkit as described in the securelist's Operation TunnelSnake report",
    "status": "experimental",
    "author": "Bhabesh Raj",
    "date": "2021/05/06",
    "modified": "2021/11/30",
    "references": [
        "https://securelist.com/operation-tunnelsnake-and-moriya-rootkit/101831"
    ],
    "tags": [
        "attack.persistence",
        "attack.privilege_escalation",
        "attack.t1543.003"
    ],
    "logsource": {
        "product": "custom-log-type1"
    },
    "detection": {
        "selection": {
            "Provider_Name": "Service Control Manager",
            "EventID": 7045,
            "ServiceName": "ZzNetSvc"
        },
        "condition": "selection"
    },
    "level": "test",
    "falsepositives": [
        "Unknown"
    ]
}

5. The error returned doesn't provide the details about the level attribute being wrong. The backend API output looks like:

{
    "error": {
        "root_cause": [
            {
                "type": "security_analytics_exception",
                "reason": "Unknown error"
            }
        ],
        "type": "security_analytics_exception",
        "reason": "Unknown error",
        "caused_by": {
            "type": "exception",
            "reason": "org.opensearch.action.search.SearchPhaseExecutionException: "
        }
    },
    "status": 400
}

What is the expected behavior?
The output of the API should return the list of cx errors which can be interpreted by the frontend to display the right behavior on the UI.

What is your host/environment?

• OS: [e.g. iOS]
• Version [e.g. 22] 2.11
• Plugins

Do you have any screenshots?
If applicable, add screenshots to help explain your problem.

Do you have any additional context?
Add any other context about the problem.