Optionally throw a NoHandlerFoundException from DispatcherServlet

Prior to this commit, the DispatcherServlet would send an error
response using the HttpServlet API when no Handler was found to
handle this request.

Now the DispatcherServlet can be configured to throw a
NoHandlerFoundException, when the throwExceptionIfNoHandlerFound
property is set to "true".
Those exceptions can be later on caught by default or custom
HandlerExceptionResolvers/ExceptionHandlers.

Issue: SPR-10481