Add check for model_dir and gitignore (#945)

* Add check for model_dir and gitignores

* code clean

* code clean

fluid/object_detection/.gitignore

-./data/pascalvoc/VOCdevkit/
-data/pascalvoc/test.txt
-data/pascalvoc/trainval.txt
+# saved model
+model/
+
 pretrained/ssd_mobilenet_v1_coco.tar.gz
 pretrained/ssd_mobilenet_v1_coco
 pretrained/mobilenet_v1_imagenet.tar.gz
 pretrained/mobilenet_v1_imagenet
+
+# coco and pascalvoc data
+data/coco/train2017/
+data/coco/train2014/
+data/coco/val2017/
+data/coco/val2014/
+data/coco/*.zip
+data/coco/annotations/
+data/pascalvoc/VOCdevkit/
+data/pascalvoc/*.tar
+data/pascalvoc/test.txt
+data/pascalvoc/trainval.txt
+
 log*
 *.log

fluid/object_detection/eval.py

@@ -96,6 +96,10 @@ if __name__ == '__main__':
     data_dir = 'data/pascalvoc'
     test_list = 'test.txt'
     label_file = 'label_list'
+
+    if not os.path.exists(args.model_dir):
+        raise ValueError("The model path [%s] does not exist." %
+                         (args.model_dir))
     if 'coco' in args.dataset:
         data_dir = './data/coco'
         if '2014' in args.dataset: