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.
Rewrote error message a bit to clarify