Add "couldn't find ffmpeg" warnings
Add "couldn't find ffmpeg" warnings

The most common issue for new pydub users is for ffmpeg/avconv to not be found resulting in a confusing error: `OSError: [Errno 2] No such file or directory`.

This warning will allow the functionality to remain the same, insuring against breaking existing code, but will at least provide some guidance to new users whose program cannot find and use the ffmpeg tool it needs
sgml/pydub,cbelth/pyMusic,miguelgrinberg/pydub,jiaaro/pydub,Geoion/pydub,joshrobo/pydub