StandardServletAsyncWebRequest handling for onError

This change ensures that an onError outcome from an async request is
also routed to onCompletion handlers registered with
StandardServletAsyncWebRequest.

Issue: SPR-13292

spring-web/src/main/java/org/springframework/web/context/request/async/StandardServletAsyncWebRequest.java

@@ -134,6 +134,7 @@ public class StandardServletAsyncWebRequest extends ServletWebRequest implements
 
 	@Override
 	public void onError(AsyncEvent event) throws IOException {
+		onComplete(event);
 	}
 
 	@Override