Determine which file an error occurs in

cherry-pick enhancement from PR #516

```
$ supervisord
Error: Unexpected end of key/value pairs in value 'KEY=val,KEY2=val,KEY3' in section 'program:foo' (file: /Users/marca/dev/git-repos/supervisor/test_conf.d/foo.conf)
For help, use /Users/marca/python/virtualenvs/supervisor/bin/supervisord -h
```

```
git cherry-pick aaecdfd71fc027f125662eb0ec1de96e344b9615

Conflicts:
	supervisor/options.py
	supervisor/tests/test_options.py
```