Add a shim Endpoint if management context is child

When management endpoints are on a different port the HandlerMappings
are restricted to a single EndpointHandlerMapping, so the error
controller (which is a normal @Controller with @RequestMappings) does
not get mapped.

Fixed by addinga shim Endpoint on "/error" that delegates to the
ErrorController (which interface picks up an extra method).

spring-boot-samples/spring-boot-sample-actuator-ui/src/test/java/org/springframework/boot/sample/ops/ui/SampleActuatorUiApplicationPortTests.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.sample.ops.ui;
 
+import static org.junit.Assert.assertEquals;
+
 import java.io.IOException;
 import java.util.Map;
 import java.util.concurrent.Callable;
@@ -25,7 +27,6 @@ import java.util.concurrent.TimeUnit;
 
 import org.junit.AfterClass;
 import org.junit.BeforeClass;
-import org.junit.Ignore;
 import org.junit.Test;
 import org.springframework.boot.SpringApplication;
 import org.springframework.context.ConfigurableApplicationContext;
@@ -35,8 +36,6 @@ import org.springframework.http.client.ClientHttpResponse;
 import org.springframework.web.client.DefaultResponseErrorHandler;
 import org.springframework.web.client.RestTemplate;
 
-import static org.junit.Assert.assertEquals;
-
 /**
  * Integration tests for separate management and main service ports.
  * 
@@ -80,9 +79,7 @@ public class SampleActuatorUiApplicationPortTests {
 	}
 
 	@Test
-	@Ignore
 	public void testMetrics() throws Exception {
-		// FIXME broken since error page is not rendered
 		@SuppressWarnings("rawtypes")
 		ResponseEntity<Map> entity = getRestTemplate().getForEntity(
 				"http://localhost:" + managementPort + "/metrics", Map.class);