message: Check return value in python tutorial
diff: @@ -25,23 +25,27 @@ import numpy as np
  import cv2 as cv
  cap = cv.VideoCapture(0)
  -
  -while(True):
  +if not cap.isOpened():
  + print("Cannot open camera")
  + exit()
  +while True:
  # Capture frame-by-frame
  ret, frame = cap.read()
  + # if frame is read correctly ret is True
  + if not ret:
  + print("Can't receive frame (stream end?). Exiting ...")
  + break
  # Our operations on the frame come here
  gray = cv.cvtColor(frame, cv.COLOR_BGR2GRAY)
  -
  # Display the resulting frame
  cv.imshow('frame', gray)
  - if cv.waitKey(1) & 0xFF == ord('q'):
  + if cv.waitKey(1) == ord('q'):
  break
  # When everything done, release the capture
  cap.release()
  -cv.destroyAllWindows()
  -@endcode
  +cv.destroyAllWindows()@endcode
  `cap.read()` returns a bool (`True`/`False`). If frame is read correctly, it will be `True`. So you can
  check end of the video by checking this return value.
  @@ -75,13 +79,17 @@ import cv2 as cv
  cap = cv.VideoCapture('vtest.avi')
  -while(cap.isOpened()):
  +while cap.isOpened():
  ret, frame = cap.read()
  + # if frame is read correctly ret is True
  + if not ret:
  + print("Can't receive frame (stream end?). Exiting ...")
  + break
  gray = cv.cvtColor(frame, cv.COLOR_BGR2GRAY)
  cv.imshow('frame', gray)
  - if cv.waitKey(1) & 0xFF == ord('q'):
  + if cv.waitKey(1) == ord('q'):
  break
  cap.release()
  @@ -125,18 +133,18 @@ cap = cv.VideoCapture(0)
  fourcc = cv.VideoWriter_fourcc(*'XVID')
  out = cv.VideoWriter('output.avi', fourcc, 20.0, (640, 480))
  -while(cap.isOpened()):
  +while cap.isOpened():
  ret, frame = cap.read()
  - if ret==True:
  + if not ret:
  + print("Can't receive frame (stream end?). Exiting ...")
  + break
  frame = cv.flip(frame, 0)
  # write the flipped frame
  out.write(frame)
  cv.imshow('frame', frame)
  - if cv.waitKey(1) & 0xFF == ord('q'):
  - break
  - else:
  + if cv.waitKey(1) == ord('q'):
  break
  # Release everything if job is finished
  