Add view rendering support for exception handling

[bclozel]

The ResponseEntityExceptionHandler handles exceptions at the MVC/WebFlux level as Problem Details. Right now, only machine-readable formats are managed.

The Spring Boot global error handling not only handles JSON, but also static pages and view templates. This infrastructure is currently based on an ErrorController and an ErrorAttributes object. Errors are handled during error dispatches.

Errors should be as much as possible handled by the application, error dispatches should be the last resort. Spring Framework should help applications by providing a flexible mechanism for rendering errors as HTML. For that, we can probably lean on content negotiation support and the ordering of handlers.