### Description

This applies to the following APIs:
- [x] open anomaly detection
- [x] start datafeed
- [x] start data frame analytics job
- [x] start trained model deployment

Currently, if the operation takes longer than the configured `timeout`, the response status code is `500`.

With Kibana in mind, a `4xx` response would be more suitable as it will lead to showing more useful information to the user about why the request failed.

Looking at [RFC 9110](https://www.rfc-editor.org/rfc/rfc9110.html#name-408-request-timeout), it seems the most appropriate `4xx` code would be `408 Request Timeout`, even though the RFC describes a different situation. 

The alternative, `504 Gateway Timeout`, would be more appropriate for this situation but it wouldn't improve the user experience in Kibana.
x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/action/TransportStartDatafeedAction.java
}
        public synchronized StoppedOrIsolated getStoppedOrIsolated() {
            return stoppedOrIsolated;
        }
        public void isolate() {
            synchronized (this) {
                
                
                
                
                if (stoppedOrIsolated == StoppedOrIsolated.NEITHER) {
                    stoppedOrIsolated = StoppedOrIsolated.ISOLATED;
                }
                if (datafeedRunner == null) {
                    return;
                }
            }
            datafeedRunner.isolateDatafeed(this);
        }
        void completeOrFailIfRequired(Exception error) {
            
            if (isCompleted()) {
                return;
            }
            if (error != null) {
                markAsFailed(error);
            } else {
                markAsCompleted();
            }
        }