This commit adds support for Spring Boot error conventions with WebFlux.
The Spring MVC support for that is based on an `Controller` that's
mapped on a specific `"/error"` path and configured as an error page in
the Servlet container. With WebFlux, this support leverages a
`WebExceptionHandler`, which catches exceptions flowing through the
reactive pipeline and handles them.

The `DefaultErrorWebExceptionHandler` supports the following:
* return a JSON error response to machine clients
* return error HTML views (templates, static or default HTML view)

One can customize the error information by contributing an
`ErrorAttributes` bean to the application context.

Spring Boot provides an `ErrorWebExceptionHandler` marker interface and a
base implementation that provides high level constructs to handle
errors, based on the Spring WebFlux functional flavor.
The error handling logic can be completely changed by providing a custom
`RouterFunction` there.

Fixes gh-8625