Add default /error view for HTML clients

* Add integration tests for /error view
* Add "error" @Bean as default view for HTML

Users may see side effects because now there will be
a ContentNegotiatingViewResolver by default for the
first time in a vanilla Actuator app. Should be
interesting.

[Fixes #54597932] [bs-273] Circular view reference for /error

spring-boot-samples/spring-boot-sample-actuator-ui/src/main/java/org/springframework/boot/sample/ops/ui/SampleActuatorUiApplication.java

@@ -40,6 +40,11 @@ public class SampleActuatorUiApplication {
 		return "home";
 	}
 
+	@RequestMapping("/foo")
+	public String foo() {
+		throw new RuntimeException("Expected exception in controller");
+	}
+
 	public static void main(String[] args) throws Exception {
 		SpringApplication.run(SampleActuatorUiApplication.class, args);
 	}