A Servlet Filter that provides an ErrorPageRegistry for non-embedded
applications (i.e. deployed WAR files). It registers error pages and handles
application errors by filtering requests and forwarding to the error pages instead of
letting the server handle them. Error pages are a feature of the servlet spec but there
is no Java API for registering them in the spec. This filter works around that by
accepting error page registrations from Spring Boot's ErrorPageRegistrar (any
beans of that type in the context will be applied to this server).