Cannot download some lectures' video file

[leonkaihao]

Describe the bug When I tried to download a full course, I found that some video lectures within the course were missing. Also, the log didn't show any download process info. Like:

[i] : Downloading chapter : (2)
[i] : Chapter (02 Kubernetes Overview)
[i] : Found (4) lecture(s).

without any download info. If I tried to download the missing video lecture, it still bypassed the video download.

To Reproduce Steps to reproduce the behavior:

1. udemy course url: [e.g https://www.udemy.com/course/learn-kubernetes]
2. Run the following command to reproduce the error.

   python udemy-dl.py https://www.udemy.com/course/learn-kubernetes -c 2 -u <user email> -p <user password> -l 3

3. See error

   
   [i] : Logged in successfully.
   [+] : Downloaded course information ..  (done)
   [i] : Trying to logout now...
   [i] : Logged out successfully.
   [+] : Course 'learn-kubernetes'
   [i] : Chapter(s) (12)
   [i] : Lecture(s) (60)

[i] : Downloading chapter : (2) [i] : Chapter (02 Kubernetes Overview) [i] : Found (4) lecture(s).

➜ udemy-dl git:(master)

**Expected behavior**
Show a progress bar to download the video file.

**Python Environment (please complete the following information):**
 - OS Platform: [macOS bigsur]
 - Python Version [3.9.5]

**Additional context**
Seems like some lectures adopt a different video encoding approach(dash?)
![image](https://user-images.githubusercontent.com/5800222/122148063-510e2180-ce9d-11eb-8e11-45c9f20cd381.png)

[mayanklalwani]

facing same issue

[reidemeister94]

Same issue here

[Lordwales]

same here also