suggested error handling

[blafasel42]

hi, after there a some issues related to images not loading, i suggest to add some error handling after:

CornerNet-Lite/core/sample/cornernet_saccade.py

Line 162 in 6a54505

image = cv2.imread(image_path)

I would add after image = cv2.imread(image_path):

        if image is None:
            raise Exception('Image could not be read', image_path)