Improve error reporting of JPEG image encoder

OpenCV issue #2604

After this patch applied an attempt to encode empty images produces exception
saying "Raw image encoder error: Empty JPEG image (DNL not supported)"

modules/highgui/test/test_grfmt.cpp

@@ -282,3 +282,12 @@ TEST(Highgui_ImreadVSCvtColor, regression)
 }
 #endif
 
+#ifdef HAVE_JPEG
+TEST(Highgui_Jpeg, encode_empty)
+{
+    cv::Mat img;
+    std::vector<uchar> jpegImg;
+
+    ASSERT_THROW(cv::imencode(".jpg", img, jpegImg), cv::Exception);
+}
+#endif