Unhelpful exception if ffmpeg is not present

Hi, Just had this error.

```
$ x-recorder start-video --display :1 --output foo.m4v

events.js:72
        throw er; // Unhandled 'error' event
              ^
Error: spawn ENOENT
    at errnoException (child_process.js:988:11)
    at Process.ChildProcess._handle.onexit (child_process.js:779:34)
```

I traced it to ffmpeg not being installed, but the exception is not very clear on this regard.
