Summary

Closes #32665

The error messages for the Task API is not stable and different types of errors can show up in different parts of the response. This change includes the entire task response in the error message so that wherever the error message is, the user can find it.

Steps to reproduce

Index data without _source:

PUT logs
{
  "mappings": {
    "_source": {
      "enabled": false
    }
  }
}

PUT logs/_doc/1
{
  "requests": {
    "count": 10,
    "foo": "bar" 
  },
  "meta": {
    "name": "Some metric",
    "description": "Some metric description", 
    "other": {
      "foo": "one", 
      "baz": "two" 
    }
  }
}

Start a reindex via the API:

$ curl localhost:5601/api/upgrade_assistant/reindex/logs -XPOST -H 'kbn-xsrf: xxx'

Wait for it to fail (should see stacktrace in ES logs) and then check the errorMessage key of the reindex status API:

$ curl localhost:5601/api/upgrade_assistant/reindex/logs