Make some methods in BasicErrorController protected

Spring MVC requires all handlers for the same path to be on the same handler
so if anyone wants to add new handlers for different content types they
have to copy a lot of code from BasicErrorController. This change increases
the visibility of the basic utility methods in BasicErrorController so that
custom handlers can be added easily.

Fixes gh-3828

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

@@ -1401,6 +1401,12 @@ the same data in HTML format (to customize it just add a `View` that resolves to
 `ErrorController` and register a bean definition of that type, or simply add a bean of
 type `ErrorAttributes` to use the existing mechanism but replace the contents.
 
+TIP: The `BasicErrorController` can be used as a base class for a custom `ErrorController`.
+This is particularly useful if you want to add a handler for a new content type (the default
+is to handle `text/html` specifically and provide a fallback for everything else). To do that
+just extend `BasicErrorController` and add a public method with a `@RequestMapping` that
+has a `produces` attribute, and create a bean of your new type.
+
 If you want more specific error pages for some conditions, the embedded servlet containers
 support a uniform Java DSL for customizing the error handling. Assuming that you have a
 mapping for `/400`: