Fixed shebangs, added error checking

samples/python2/common.py

@@ -1,14 +1,14 @@
-#/usr/bin/env python
-
 '''
 This module contains some common routines used by other samples.
 '''
 
 import numpy as np
 import cv2
+
+# built-in modules
 import os
-from contextlib import contextmanager
 import itertools as it
+from contextlib import contextmanager
 
 image_extensions = ['.bmp', '.jpg', '.jpeg', '.png', '.tif', '.tiff', '.pbm', '.pgm', '.ppm']