More descriptive error when mplayer is not installed

On a vanilla Sierra install, I ran `npm install -g npr-one` and then got the following:

```
connecting to npr one...
events.js:160
throw er; // Unhandled 'error' event
^
```

The solution was to run `brew install mplayer`. It'd be nice if the CLI could return with a more descriptive error when mplayer isn't installed.

Start at the CLI entry point that launches mplayer and reproduce the failure on a vanilla Sierra install without mplayer installed. Done means the CLI reports that mplayer is missing and tells the user to install it, instead of exposing the unhandled error.

Written by the indexing model from the issue text.