Don't fail if the video thumbnail couldn't be downloaded (fixes #1516)

Just report a warning

youtube_dl/YoutubeDL.py

@@ -545,11 +545,15 @@ class YoutubeDL(object):
                 thumb_filename = filename.rpartition('.')[0] + u'.' + thumb_format
                 self.to_screen(u'[%s] %s: Downloading thumbnail ...' %
                                (info_dict['extractor'], info_dict['id']))
+                try:
                     uf = compat_urllib_request.urlopen(info_dict['thumbnail'])
                     with open(thumb_filename, 'wb') as thumbf:
                         shutil.copyfileobj(uf, thumbf)
                     self.to_screen(u'[%s] %s: Writing thumbnail to: %s' %
                         (info_dict['extractor'], info_dict['id'], thumb_filename))
+                except (compat_urllib_error.URLError, compat_http_client.HTTPException, socket.error) as err:
+                    self.report_warning(u'Unable to download thumbnail "%s": %s' %
+                        (info_dict['thumbnail'], compat_str(err)))
 
         if not self.params.get('skip_download', False):
             if self.params.get('nooverwrites', False) and os.path.exists(encodeFilename(filename)):