Validate files passed to --config option

Here's a quick implementation to address benoitc/gunicorn#469.  I apologize for not understanding pytest well enough to provide unit test coverage for it.

After the patch:

```
(venv)marmida@monolith:~/develop/gunicorn$ venv/bin/gunicorn -c path/doesnt/exist.py webapp:app

Error: No such file: 'path/doesnt/exist.py'
```