ConstructorResolver's exception message on null factory-bean hints at potential BeanPostProcessor involvement

Issue: SPR-11951
(cherry picked from commit 6c41cc3)

spring-beans/src/main/java/org/springframework/beans/factory/support/ConstructorResolver.java

@@ -377,7 +377,7 @@ class ConstructorResolver {
 			factoryBean = this.beanFactory.getBean(factoryBeanName);
 			if (factoryBean == null) {
 				throw new BeanCreationException(mbd.getResourceDescription(), beanName,
-						"factory-bean '" + factoryBeanName + "' returned null");
+						"factory-bean '" + factoryBeanName + "' (or a BeanPostProcessor involved) returned null");
 			}
 			factoryClass = factoryBean.getClass();
 			isStatic = false;