stagefright: httplive: Propagate error code from fetchPlaylist

LiveSession::fetchPlaylist returns NULL if any fetch error occurs.
If the 'bytesRead' argument is non-NULL, return the number bytes read
or the error code so that the caller can handle recoverable errors.

CRs-Fixed: 759588
Change-Id: I68e29fb846a47f865146192c24a27864610bc9c0