REPROC_EVENT_ERR is never set (at least in the example poll.c)

This [line](https://github.com/DaanDeMeyer/reproc/blob/main/reproc/examples/poll.c#L57) never has ```REPROC_EVENT_ERR``` in ```children[i].events```, when .interests = ```REPROC_EVENT_OUT|REPROC_EVENT_ERR|REPROC_EVENT_EXIT```.
