Expose exception as request attribute when calling sendError

When sending an error response, expose the encountered exception
as a "org.springframework.web.util.WebUtils.EXCEPTION" request
attribute.

This request attribute is similar to the standard servlet
"javax.servlet.error.exception" request attribute, but is more
flexible since the response won't be automatically handled
by the servlet container as an INTERNAL_SERVER_ERROR error page.

Issue: SPR-11686