Add documentation about supported sites (Fixes #4503)

youtube_dl/extractor/generic.py

@@ -23,6 +23,7 @@ from ..utils import (
     unescapeHTML,
     unified_strdate,
     unsmuggle_url,
+    UnsupportedError,
     url_basename,
 )
 from .brightcove import BrightcoveIE
@@ -1057,7 +1058,7 @@ class GenericIE(InfoExtractor):
                     'url': new_url,
                 }
         if not found:
-            raise ExtractorError('Unsupported URL: %s' % url)
+            raise UnsupportedError(url)
 
         entries = []
         for video_url in found:

youtube_dl/utils.py

@@ -464,6 +464,13 @@ class ExtractorError(Exception):
         return ''.join(traceback.format_tb(self.traceback))
 
 
+class UnsupportedError(ExtractorError):
+    def __init__(self, url):
+        super(UnsupportedError, self).__init__(
+            'Unsupported URL: %s' % url, expected=True)
+        self.url = url
+
+
 class RegexNotFoundError(ExtractorError):
     """Error when a regex didn't match"""
     pass