Revisit how we return ES error in the <SectionError />

[sebelga]

In index template, after the upgrade to composable template, we don't get useful errors coming back from ES when the template is invalid. Instead we get a

[illegal_argument_exception] composable template [abc] template after composition is invalid

Which is really not that great to fix the template.

The reason is that ES returns the reason under a caused_by, which we strip in our <SectionError /> component. We need to retrieve recursively all the leaves of caused_by.reason to get meaningful error messages.

This is the complete error returned by ES not surfaced in the UI

{
  "msg": "[illegal_argument_exception] composable template [abc] template after composition is invalid",
  "path": "/_index_template/abc",
  "query": {},
  "body": {
    ...
  },
  "statusCode": 400,
  "response": {
    "error": {
      "root_cause": [
        {
          "type": "illegal_argument_exception",
          "reason": "composable template [abc] template after composition is invalid"
        }
      ],
      "type": "illegal_argument_exception",
      "reason": "composable template [abc] template after composition is invalid",
      "caused_by": {
        "type": "illegal_argument_exception",
        "reason": "invalid composite mappings for [abc]",
        "caused_by": {
          "type": "mapper_parsing_exception",
          "reason": "Failed to parse mapping: analyzer [customAnalyzer_1] not found for field [test]",
          "caused_by": {
            "type": "mapper_parsing_exception",
            "reason": "analyzer [customAnalyzer_1] not found for field [test]"
          }
        }
      }
    },
    "status": 400
  }
}

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)