Try all possible mediaselectors even if one succeeds

Some mediaselectors may be parseable but have no formats (eg captions only).

This revision tries all possible mediaselectors and assembles the formats and subtitles found.

If any formats are found, the known exceptions ('notukerror', 'geolocation', 'selectionunavailable') are not propagated.

youtube_dl/extractor/bbc.py

@@ -350,16 +350,26 @@ class BBCCoUkIE(InfoExtractor):
 
     def _download_media_selector(self, programme_id):
         last_exception = None
+        formats = []
+        subtitles = []
+        # as some mediaselectors may be parseable but have 
+        # no formats (eg captions only), try all possible
+        # mediaselectors
         for mediaselector_url in self._MEDIASELECTOR_URLS:
             try:
-                return self._download_media_selector_url(
+                formatsAndSubtitles = self._download_media_selector_url(
                     mediaselector_url % programme_id, programme_id)
+                formats += formatsAndSubtitles[0]
+                subtitles += formatsAndSubtitles[1]
             except BBCCoUkIE.MediaSelectionError as e:
                 if e.id in ('notukerror', 'geolocation', 'selectionunavailable'):
                     last_exception = e
                     continue
                 self._raise_extractor_error(e)
-        self._raise_extractor_error(last_exception)
+        # ignore a trapped exception if formats were found 
+        if last_exception and not formats:
+            self._raise_extractor_error(last_exception)
+        return formats, subtitles
 
     def _download_media_selector_url(self, url, programme_id=None):
         media_selection = self._download_xml(