Now raises an FPM error for missing source dir

Current behaviour:

```bash
$ fpm -s dir -t deb -n foo -a all -v 0.1 --prefix /opt /fakesourcedir
/opt/boxen/rbenv/versions/1.9.3-p448/lib/ruby/1.9.1/find.rb:38:in `block in find': No such file or directory (Errno::ENOENT)
	from /opt/boxen/rbenv/versions/1.9.3-p448/lib/ruby/1.9.1/find.rb:38:in `collect!'
#etc. etc.
```

New behaviour:
```bash
$ fpm -s dir -t deb -n foo -a all -v 0.1 --prefix /opt /fakesourcedir
Source directory does not exist: /fakesourcedir {:level=>:fatal}
```

However, I'm having difficulty with the specs, I'm not sure how to fix the ones I've broken and how to add one for this specific case? Help on that would be appreciated! 

@jordansissel any thoughts? :+1: