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

Previously when running the tests for spring-web, spring-webmvc,
spring-test-mvc, and spring-orm there were possible collissions in the
classpath due to the fact that each had its own copy of the web.Mock*
classes in it. This causes uncertainty for which class the code was
running against. Furthermore, the maintance of keeping the copies up to
date was tedious.

Now there are two copies of the web.Mock* classes. The ones that exist
in spring-test main sources and a copy that supports servlet 3 within
spring-web test sources. The copy in spring-web test sources has been
moved to a new package to avoid having the classes exist twice on the
classpath within Eclipse.

DefaultHandlerExceptionResolver and ResponseEntityExceptionHandler now
both set the "javax.servlet.error.exception" request attribute to the
raised exception, allowing custom error pages configured via web.xml.

Issue: SPR-9653

Rename ExceptionHandlerSupport to ResponseEntityExceptionHandler and
emphasize the contrast to DefaultHandlerExceptionResovler -- i.e.
one returns a ResponseEntity and relies on message converters while
the other returns a ModelAndView and relies on view resolution.

Issue: SPR-9290

The new class is functionally equivalent to the
DefaultHandlerExceptionResolver (i.e. it translates Spring MVC
exceptions to various status codes) but uses an @ExceptionHandler
returning a ResponseEntity<Object>, which means it can be customized to
write error content to the body of the response.

Issue: SPR-9290