Determine which file an error occurs in

I found a way to display what file an error occurred in, by doing some further extension of [`UnhosedConfigParser`](https://github.com/Supervisor/supervisor/pull/516/files#diff-0240b66c1a20db854f3d6eacb71caa32L1657).

Example:

```
$ supervisord --nodaemon -c supervisor.conf
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
```