[ted] fixed error in case of no subtitles present

I created a test, but I leave it commented since TED videos get
new subtitles frequently.

test/test_ted_subtitles.py

@@ -51,6 +51,12 @@ def test_automatic_captions(self):
         self.DL.params['subtitleslang'] = ['en']
         subtitles = self.getSubtitles()
         self.assertTrue(len(subtitles.keys()) == 0)
+    # def test_nosubtitles(self):
+    #     self.DL.expect_warning(u'video doesn\'t have subtitles')
+    #     self.url = 'http://www.ted.com/talks/rodrigo_canales_the_deadly_genius_of_drug_cartels.html'
+    #     self.DL.params['writesubtitles'] = True
+    #     self.DL.params['allsubtitles'] = True
+    #     subtitles = self.getSubtitles()
     def test_multiple_langs(self):
         self.DL.params['writesubtitles'] = True
         langs = ['es', 'fr', 'de']

youtube_dl/extractor/ted.py

@@ -3,6 +3,11 @@
 
 from .subtitles import SubtitlesInfoExtractor
 
+from ..utils import (
+    compat_str,
+    RegexNotFoundError,
+)
+
 class TEDIE(SubtitlesInfoExtractor):
     _VALID_URL=r'''http://www\.ted\.com/
                    (
@@ -105,6 +110,7 @@ def _talk_info(self, url, video_id=0):
         return info
 
     def _get_available_subtitles(self, video_id, webpage):
+        try:
             options = self._search_regex(r'(?:<select name="subtitles_language_select" id="subtitles_language_select">)(.*?)(?:</select>)', webpage, 'subtitles_language_select', flags=re.DOTALL)
             languages = re.findall(r'(?:<option value=")(\S+)"', options)
             if languages:
@@ -113,4 +119,6 @@ def _get_available_subtitles(self, video_id, webpage):
                     url = 'http://www.ted.com/talks/subtitles/id/%s/lang/%s/format/srt' % (video_id, l)
                     sub_lang_list[l] = url
                 return sub_lang_list
+        except RegexNotFoundError as err:
+            self._downloader.report_warning(u'video doesn\'t have subtitles')
         return {}