In order to remove any confusion if the user has duplicate `renderd` sections in their `renderd.conf` file.
Currently with a configuration like this one:
```ini
[renderd]
pid_file=/run/renderd/renderd.pid
socketname=/run/renderd/renderd.sock
stats_file=/run/renderd/renderd.stats
tile_dir=/var/cache/renderd/tiles
[renderd0]
pid_file=/run/renderd/renderd0.pid
socketname=/run/renderd/renderd0.sock
stats_file=/run/renderd/renderd0.stats
tile_dir=/var/cache/renderd/tiles
```
only the second `renderd` section (`renderd0`) will actually be used.
After this patch is merged, an error message will be shown and the application will exit.
