Implement MessagingException.toString()

Issue: SPR-15091
Closes gh-1284

spring-messaging/src/main/java/org/springframework/messaging/MessagingException.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -66,4 +66,10 @@ public class MessagingException extends NestedRuntimeException {
 		return this.failedMessage;
 	}
 
+	@Override
+	public String toString() {
+		return super.toString() + (this.failedMessage == null ? ""
+				: (", failedMessage=" + this.failedMessage));
+	}
+
 }