[youtube] Fix 'liveChatReplayContinuationData' missing 'continuation' key

live_chat_continuation['continuations'][0]['liveChatReplayContinuationData']['continuation'] can not exist.
So catch the KeyError.

Traceback:
$ tubeup 'https://youtube.com/watch?v=JyE9OF03cao'
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dlc version 2020.10.25
[debug] Python version 3.7.3 (CPython) - Linux-5.8.0-0.bpo.2-amd64-x86_64-with-debian-10.6
[debug] exe versions: ffmpeg 3.3.9, ffprobe 3.3.9
[debug] Proxy map: {}
There are no annotations to write.
[download] 452.59KiB at 615.35KiB/s (00:01)ERROR: 'liveChatReplayContinuationData'
Traceback (most recent call last):
  File "/mnt/data2/Backup/Wiki/.local/lib/python3.7/site-packages/youtube_dlc/YoutubeDL.py", line 846, in extract_info
    return self.process_ie_result(ie_result, download, extra_info)
  File "/mnt/data2/Backup/Wiki/.local/lib/python3.7/site-packages/youtube_dlc/YoutubeDL.py", line 901, in process_ie_result
    return self.process_video_result(ie_result, download=download)
  File "/mnt/data2/Backup/Wiki/.local/lib/python3.7/site-packages/youtube_dlc/YoutubeDL.py", line 1696, in process_video_result
    self.process_info(new_info)
  File "/mnt/data2/Backup/Wiki/.local/lib/python3.7/site-packages/youtube_dlc/YoutubeDL.py", line 1894, in process_info
    dl(sub_filename, sub_info, subtitle=True)
  File "/mnt/data2/Backup/Wiki/.local/lib/python3.7/site-packages/youtube_dlc/YoutubeDL.py", line 1866, in dl
    return fd.download(name, info, subtitle)
  File "/mnt/data2/Backup/Wiki/.local/lib/python3.7/site-packages/youtube_dlc/downloader/common.py", line 375, in download
    return self.real_download(filename, info_dict)
  File "/mnt/data2/Backup/Wiki/.local/lib/python3.7/site-packages/youtube_dlc/downloader/youtube_live_chat.py", line 85, in real_download
    continuation_id = live_chat_continuation['continuations'][0]['liveChatReplayContinuationData']['continuation']
KeyError: 'liveChatReplayContinuationData'

youtube_dlc/downloader/youtube_live_chat.py

@@ -82,7 +82,10 @@ class YoutubeLiveChatReplayFD(FragmentFD):
                         offset = int(replay_chat_item_action['videoOffsetTimeMsec'])
                     processed_fragment.extend(
                         json.dumps(action, ensure_ascii=False).encode('utf-8') + b'\n')
-                continuation_id = live_chat_continuation['continuations'][0]['liveChatReplayContinuationData']['continuation']
+                try:
+                    continuation_id = live_chat_continuation['continuations'][0]['liveChatReplayContinuationData']['continuation']
+                except KeyError:
+                    continuation_id = None
 
             self._append_fragment(ctx, processed_fragment)