fix: handle youtube TranscriptsDisabled (#3276)

handles error when youtube video has transcripts disabled

```
youtube_transcript_api._errors.TranscriptsDisabled: 
Could not retrieve a transcript for the video https://www.youtube.com/watch?v=<URL> This is most likely caused by:

Subtitles are disabled for this video

If you are sure that the described cause is not responsible for this error and that a transcript should be retrievable, please create an issue at https://github.com/jdepoix/youtube-transcript-api/issues. Please add which version of youtube_transcript_api you are using and provide the information needed to replicate the error. Also make sure that there are no open issues which already describe your problem!
```

Signed-off-by: Sertac Ozercan <sozercan@gmail.com>

langchain/document_loaders/youtube.py

@@ -114,7 +114,11 @@ class YoutubeLoader(BaseLoader):
     def load(self) -> List[Document]:
         """Load documents."""
         try:
-            from youtube_transcript_api import NoTranscriptFound, YouTubeTranscriptApi
+            from youtube_transcript_api import (
+                NoTranscriptFound,
+                TranscriptsDisabled,
+                YouTubeTranscriptApi,
+            )
         except ImportError:
             raise ImportError(
                 "Could not import youtube_transcript_api python package. "
@@ -129,7 +133,11 @@ class YoutubeLoader(BaseLoader):
             video_info = self._get_video_info()
             metadata.update(video_info)
 
-        transcript_list = YouTubeTranscriptApi.list_transcripts(self.video_id)
+        try:
+            transcript_list = YouTubeTranscriptApi.list_transcripts(self.video_id)
+        except TranscriptsDisabled:
+            return []
+
         try:
             transcript = transcript_list.find_transcript([self.language])
         except NoTranscriptFound: