Fix #5 by adding 'xerror' and 'stderr' psuedo err_detect options

[crass]

This is dependent on PR #19. Really this PR is just the last 2 commits. It also fixes issue #5.

The xerror option adds the -xerror option to ffmpeg which causes it to exit early on any corrupt frames. The stderr option causes the ffmpeg_check call to fail if there is any error level log messages.

This was implemented as psuedo options to err_detect instead of a new option to have all ffmpeg error handling options go through one option. Also, even though ffmpeg's -err_detect option has no effect for me, even on the video files with corruption that I've tried, I've left this capability as it might be working for others.