Update docs with DispatcherType.ERROR for filters

Some frameworks handle all requests in a Filter, so you have to
explicitly register it as an ERROR dispatcher.

See gh-1272

Author: Dave Syer

spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -1320,7 +1320,7 @@ in the ``Production-ready features'' section.
 
 [[howto-customize-the-whitelabel-error-page]]
 === Customize the ``whitelabel'' error page
-The Actuator installs a ``whitelabel'' error page that you will see in browser client if
+Spring Boot installs a ``whitelabel'' error page that you will see in browser client if
 you encounter a server error (machine clients consuming JSON and other media types should
 see a sensible response with the right error code). To switch it off you can set
 `error.whitelabel.enabled=false`, but normally in addition or alternatively to that you
@@ -1333,6 +1333,8 @@ of the default configuration you should find a `BeanNameViewResolver` in your
 `ApplicationContext` so a `@Bean` with id `error` would be a simple way of doing that.
 Look at {sc-spring-boot-autoconfigure}/web/ErrorMvcAutoConfiguration.{sc-ext}[`ErrorMvcAutoConfiguration`] for more options.
 
+See also the section on <<boot-features-error-handling, Error Handling>> for details of
+how to register handlers in the servlet container.
 
 
 [[howto-security]]

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -998,6 +998,24 @@ You can also use regular Spring MVC features like http://docs.spring.io/spring/d
 methods] and http://docs.spring.io/spring/docs/current/spring-framework-reference/htmlsingle/#mvc-ann-controller-advice[`@ControllerAdvice`].
 The `ErrorController` will then pick up any unhandled exceptions.
 
+N.B. if you register an `ErrorPage` with a path that will end up being handled by a `Filter` (e.g. as is common with some non-Spring web frameworks, 
+like Jersey and Wicket), then the `Filter` has to be explicitly registered as an `ERROR` dispatcher, e.g.
+
+[source,java,indent=0,subs="verbatim,quotes,attributes"]
+----
+@Bean
+public FilterRegistrationBean myFilter() {
+
+  FilterRegistrationBean registration = new FilterRegistrationBean();
+  registration.setFilter(new MyFilter());
+  ...
+  registration.setDispatcherTypes(EnumSet.allOf(DispatcherType.class));
+  return registration;
+
+}
+----
+
+(the default `FilterRegistrationBean` does not include the `ERROR` dispatcher type).
 
 
 [[boot-features-embedded-container]]