non 0 status on error
p8tool should exit with non-zero status when error occurs while processing a cart.

Example: 
![10521 p8](https://cloud.githubusercontent.com/assets/4397533/16170620/3fa2980a-3559-11e6-839b-6b879d56e83d.png)
`p8tool stats` returns 

```
/home/teddy/.lexaloffle/pico-8/bbs/1/10521.p8.png: Expected end at line 474 char 31
/home/teddy/.lexaloffle/pico-8/bbs/1/10521.p8.png: could not load cart
```

on this cart but exit status is 0 
This makes using p8tool in shell scripts quiet hard.
