Add convention based status error pages

Update `spring-boot-sample-web-mustache` to show how convention based
status error pages can be used.

See gh-2691

spring-boot-samples/spring-boot-sample-web-mustache/src/main/java/sample/mustache/WelcomeController.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.
@@ -20,8 +20,10 @@ import java.util.Date;
 import java.util.Map;
 
 import org.springframework.beans.factory.annotation.Value;
+import org.springframework.http.HttpStatus;
 import org.springframework.stereotype.Controller;
 import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.ResponseStatus;
 
 @Controller
 public class WelcomeController {
@@ -36,4 +38,29 @@ public class WelcomeController {
 		return "welcome";
 	}
 
+	@RequestMapping("/serviceUnavailable")
+	public String ServiceUnavailable() {
+		throw new ServiceUnavailableException();
+	}
+
+	@RequestMapping("/bang")
+	public String bang() {
+		throw new RuntimeException("Boom");
+	}
+
+	@RequestMapping("/insufficientStorage")
+	public String insufficientStorage() {
+		throw new InsufficientStorageException();
+	}
+
+	@ResponseStatus(HttpStatus.SERVICE_UNAVAILABLE)
+	private static class ServiceUnavailableException extends RuntimeException {
+
+	}
+
+	@ResponseStatus(HttpStatus.INSUFFICIENT_STORAGE)
+	private static class InsufficientStorageException extends RuntimeException {
+
+	}
+
 }