Unwrap CompletionException in return value handler

See gh-22476

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/DeferredResultMethodReturnValueHandler.java

 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,6 +16,7 @@
 
 package org.springframework.web.servlet.mvc.method.annotation;
 
+import java.util.concurrent.CompletionException;
 import java.util.concurrent.CompletionStage;
 import java.util.function.BiFunction;
 
@@ -93,6 +94,9 @@ public class DeferredResultMethodReturnValueHandler implements HandlerMethodRetu
 		DeferredResult<Object> result = new DeferredResult<>();
 		future.handle((BiFunction<Object, Throwable, Object>) (value, ex) -> {
 			if (ex != null) {
+				if (ex instanceof CompletionException && ex.getCause() != null) {
+					ex = ex.getCause();
+				}
 				result.setErrorResult(ex);
 			}
 			else {