Allow URL video parsing when a warning is triggered by youtube-dl

Summary:
Currently, when a video URL is given on a channel with the parse-url
flag enabled, youtube-dl is called to resolve video URLS for video
domains.

If any output reaches stderr, exec throws an error, intercepted
by the catch block, and nothing happens.

Fixes T1234.

Thanks to @xcombelle to have suggested the use of --no-warning flag.

Test Plan: url:getvideotitle https://www.youtube.com/watch?v=4CXOb0Oubxo

Reviewers: xcombelle

Reviewed By: xcombelle

Subscribers: xcombelle

Maniphest Tasks: T1234

Differential Revision: https://devcentral.nasqueron.org/D1017