More info in "Unexpected end of key/value pairs" error

Previously it was hard to know where the error was coming from if you had a large number of include files. E.g.:

```
$ supervisord
Error: Unexpected end of key/value pairs
For help, use /usr/local/bin/supervisord -h
```

This makes it easier by printing the string value and the section in which it occurred. E.g.:

```
$ supervisord --nodaemon -c supervisor.conf
Error: Unexpected end of key/value pairs in value 'KEY=val,KEY2=val,KEY3' in section 'program:hello'
For help, use /Users/marca/python/virtualenvs/supervisor/bin/supervisord -h
```