Error if YAML files are missing

I ran into cryptic error when running `bundle exec opsworks deploy staging`

```
$ bundle exec opsicle deploy staging
error: missing required option stack_id. Use --trace to view backtrace
```

Turns out I wasn't in the repo root, and YAML.load_file() doesn't signal that there was no file to load.

The issue was entirely my fault, but I still thought it would be cool if it was a little more helpful.