Add /error to ignored paths for security autoconfig

Protecting /error doesn't make a great deal of sense and if it is
protected you don't get the ErrorPageFilter for the attempt at loading
it, so Tomcat renders its own HTML error page (when deployed as WAR).

Fixes gh-1548

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/autoconfigure/ManagementSecurityAutoConfigurationTests.java

@@ -74,7 +74,7 @@ public class ManagementSecurityAutoConfigurationTests {
 		this.context.refresh();
 		assertNotNull(this.context.getBean(AuthenticationManagerBuilder.class));
 		// 6 for static resources, one for management endpoints and one for the rest
-		assertEquals(8, this.context.getBean(FilterChainProxy.class).getFilterChains()
+		assertEquals(9, this.context.getBean(FilterChainProxy.class).getFilterChains()
 				.size());
 	}
 
@@ -144,7 +144,7 @@ public class ManagementSecurityAutoConfigurationTests {
 		this.context.refresh();
 		// Just the management endpoints (one filter) and ignores now plus the backup
 		// filter on app endpoints
-		assertEquals(8, this.context.getBean(FilterChainProxy.class).getFilterChains()
+		assertEquals(9, this.context.getBean(FilterChainProxy.class).getFilterChains()
 				.size());
 	}