Polish serialization/deserialization exception logging

This commit introduces the following changes in AbstractHandlerExceptionResolver:
 - warnLogger used to log exception is enabled by default
 - the exception message is now logged instead of the whole exception stacktrace
 - warn logging is only performed if doResolveException() returns a non-null
   ModelAndView, in order to avoid logging multiple times the error

Issue: SPR-13100

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/ResponseEntityExceptionHandlerTests.java

@@ -56,6 +56,7 @@ import org.springframework.web.servlet.mvc.multiaction.NoSuchRequestHandlingMeth
 import org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver;
 
 import static org.junit.Assert.*;
+import org.mockito.Mockito;
 
 /**
  * Test fixture for {@link ResponseEntityExceptionHandler}.
@@ -179,7 +180,7 @@ public class ResponseEntityExceptionHandlerTests {
 
 	@Test
 	public void methodArgumentNotValid() {
-		Exception ex = new MethodArgumentNotValidException(null, null);
+		Exception ex = Mockito.mock(MethodArgumentNotValidException.class);
 		testException(ex);
 	}