Fall back to ffmpeg instead of ffprobe

[phihag]

Some users (notably on Mac OSX) do have ffmpeg, but not ffprobe. We should simply call

ffmpeg -i file -invalid

(or something better, if possible) in these cases.

[ubitux]

Note that contrary to ffmpeg, ffprobe has various outputs such as JSON, XML, CSV, "flat", etc. which might be more stable (and friendly) than parsing ffmpeg output. See http://ffmpeg.org/ffprobe.html#Writers for more information.