diagnose View Rendering exceptions in DispatcherServlet.render()

SPR-10340 proposed fix

Diagnose View-rendering, JSP exceptions etc in DispatcherServlet.render(). There is already existing DEBUG logging before rendering the view, so it is a minor & sensible improvement to log any failure that then occurs.

This is logged at DEBUG level to avoid interfering with existing production logging, but provide necessary diagnostic information during software development/ engineering.