Changed the error message.

I changed the ExtractorError from ```msg = msg + u'; please report this issue on http://yt-dl.org/bug'``` to ```msg = msg + u'; please report this issue on http://yt-dl.org/bug with the complete output by running the same command with --verbose flag'```
Hopefully this will tell the users to report bugs with the complete output.

youtube_dl/utils.py

@@ -474,7 +474,7 @@ class ExtractorError(Exception):
         """ tb, if given, is the original traceback (so that it can be printed out). """
 
         if not sys.exc_info()[0] in (compat_urllib_error.URLError, socket.timeout, UnavailableVideoError):
-            msg = msg + u'; please report this issue on http://yt-dl.org/bug'
+            msg = msg + u'; please report this issue on http://yt-dl.org/bug with the complete output by running the same command with --verbose flag'
         super(ExtractorError, self).__init__(msg)
 
         self.traceback = tb