libhb: check AVIOContext.error for errors while muxing
libavformat doesn't always check this, so the return code from
av_interleaved_write_frame doesn't indicate an error when there
is a failure to write to disk.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5850 b64f7644-9d1e-0410-96f1-a4d463321fa5