Allow ExitCodeGenerator to be used on Exceptions

Update exit code support to allow the ExitCodeGenerator interface to
be placed on an Exception. Any uncaught exception implementing the
interface and returning a non `0` status will now trigger a System.exit
with the code.

Fixes gh-4803

spring-boot-samples/spring-boot-sample-batch/src/main/java/sample/batch/SampleBatchApplication.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2015 the original author or authors.
+ * Copyright 2012-2016 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.
@@ -42,6 +42,7 @@ public class SampleBatchApplication {
 
 	@Bean
 	protected Tasklet tasklet() {
+
 		return new Tasklet() {
 			@Override
 			public RepeatStatus execute(StepContribution contribution,
@@ -49,6 +50,7 @@ public class SampleBatchApplication {
 				return RepeatStatus.FINISHED;
 			}
 		};
+
 	}
 
 	@Bean