[YouTube] Thumbnail download fails completely if maxresdefault.jpg is not available

[SebiderSushi]

Checklist

• I'm reporting a broken site support
• I've verified that I'm running youtube-dl version 2019.06.27
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• I've searched the bugtracker for similar issues including closed ones

Verbose log

$ youtube-dl --skip-download --write-thumbnail -v MTJHdKsmuxU 0uEpX2elNMQ lAW
Cxqk7ZzY 5UIYSgtj59w sgx8OL5jrEM 8kDAt7h5lO8
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'--skip-download', u'--write-thumbnail', u'-v', u'MTJHdKsmuxU', u'0uEpX2elNMQ', u'lAWCxqk7ZzY', u'5UIYSgtj59w', u'sgx8OL5jrEM', u'8kDAt7h5lO8']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2019.06.27
[debug] Python version 2.7.12 (CPython) - Linux-4.15.0-52-generic-x86_64-with-Ubuntu-16.04-xenial
[debug] exe versions: ffmpeg 2.8.15, ffprobe 2.8.15, rtmpdump 2.4
[debug] Proxy map: {}
[youtube] MTJHdKsmuxU: Downloading webpage
[youtube] MTJHdKsmuxU: Downloading video info webpage
[debug] Default format spec: bestvideo+bestaudio/best
[youtube] MTJHdKsmuxU: Downloading thumbnail ...
WARNING: Unable to download thumbnail "https://i.ytimg.com/vi/MTJHdKsmuxU/maxresdefault.jpg": HTTP Error 404: Not Found
[youtube] 0uEpX2elNMQ: Downloading webpage
[youtube] 0uEpX2elNMQ: Downloading video info webpage
[debug] Default format spec: bestvideo+bestaudio/best
[youtube] 0uEpX2elNMQ: Downloading thumbnail ...
WARNING: Unable to download thumbnail "https://i.ytimg.com/vi/0uEpX2elNMQ/maxresdefault.jpg": HTTP Error 404: Not Found
[youtube] lAWCxqk7ZzY: Downloading webpage
[youtube] lAWCxqk7ZzY: Downloading video info webpage
[debug] Default format spec: bestvideo+bestaudio/best
[youtube] lAWCxqk7ZzY: Downloading thumbnail ...
WARNING: Unable to download thumbnail "https://i.ytimg.com/vi/lAWCxqk7ZzY/maxresdefault.jpg": HTTP Error 404: Not Found
[youtube] 5UIYSgtj59w: Downloading webpage
[youtube] 5UIYSgtj59w: Downloading video info webpage
[debug] Default format spec: bestvideo+bestaudio/best
[youtube] 5UIYSgtj59w: Downloading thumbnail ...
WARNING: Unable to download thumbnail "https://i.ytimg.com/vi/5UIYSgtj59w/maxresdefault.jpg": HTTP Error 404: Not Found
[youtube] sgx8OL5jrEM: Downloading webpage
[youtube] sgx8OL5jrEM: Downloading video info webpage
[debug] Default format spec: bestvideo+bestaudio/best
[youtube] sgx8OL5jrEM: Downloading thumbnail ...
WARNING: Unable to download thumbnail "https://i.ytimg.com/vi/sgx8OL5jrEM/maxresdefault.jpg": HTTP Error 404: Not Found
[youtube] 8kDAt7h5lO8: Downloading webpage
[youtube] 8kDAt7h5lO8: Downloading video info webpage
[debug] Default format spec: bestvideo+bestaudio/best
[youtube] 8kDAt7h5lO8: Downloading thumbnail ...
WARNING: Unable to download thumbnail "https://i.ytimg.com/vi/8kDAt7h5lO8/maxresdefault.jpg": HTTP Error 404: Not Found

Description

The Problem:

In the playlist https://www.youtube.com/playlist?list=PL94C7EDE446C02090 i encountered 6 playable videos where youtube-dl failed to download a thumbnail.
Their IDs are:

MTJHdKsmuxU
0uEpX2elNMQ
lAWCxqk7ZzY
5UIYSgtj59w
sgx8OL5jrEM
8kDAt7h5lO8

As seen in the error log, youtube-dl tries to download the maxresdefault.jpg for these IDs but fails since the correct file in these cases would actually be hqdefault.jpg. (Also found in the HTML page source code, contrary to maxresdefault.jpg)
https://i.ytimg.com/vi/MTJHdKsmuxU/hqdefault.jpg
https://i.ytimg.com/vi/0uEpX2elNMQ/hqdefault.jpg
https://i.ytimg.com/vi/lAWCxqk7ZzY/hqdefault.jpg
https://i.ytimg.com/vi/5UIYSgtj59w/hqdefault.jpg
https://i.ytimg.com/vi/sgx8OL5jrEM/hqdefault.jpg
https://i.ytimg.com/vi/8kDAt7h5lO8/hqdefault.jpg

Possible solution

Either fix a bug that leads to youtube-dl getting an incorrect thumbnail link or when maxresdefault.jpg can't be found, try other names like hqdefault.jpg automatically instead of failing completely.