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-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/MainMethodRunner.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2013 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.
@@ -16,6 +16,7 @@
 
 package org.springframework.boot.loader;
 
+import java.lang.Thread.UncaughtExceptionHandler;
 import java.lang.reflect.Method;
 
 /**
@@ -53,7 +54,14 @@ public class MainMethodRunner implements Runnable {
 			mainMethod.invoke(null, new Object[] { this.args });
 		}
 		catch (Exception ex) {
-			ex.printStackTrace();
+			UncaughtExceptionHandler handler = Thread.currentThread()
+					.getUncaughtExceptionHandler();
+			if (handler != null) {
+				handler.uncaughtException(Thread.currentThread(), ex);
+			}
+			else {
+				ex.printStackTrace();
+			}
 			System.exit(1);
 		}
 	}