Wrong error message when create AD trigge

[ylwu-amzn]

https://discuss.opendistrocommunity.dev/t/cannot-create-trigger-from-anomaly-detector-requesttimeout/4832

Alerting create AD trigger page will call preview detector API to get sample anomaly results and detector feature configuration. The preview API may not work for some cases

1. If preview detector API timeout or throw error, AD trigger page can’t get any data, then it will show this warning “No features have been added to this anomaly detector. A feature is a metric that is used for anomaly detection. A detector can discover anomalies across one or more features.”. The message is not so accurate for such case as detector have features but just failed to call preview API.

2. When add AD trigger, alerting frontend will call preview API with last 5 days data. And preview API needs 400+ data points to generate sample anomaly results. If the anomaly detector interval is too big or there is no enough data in last 5 days, then preview API won’t return anything, then we will see this error message.

We need to tune the error message.