This seems to be a pretty common switch, and it looks like the case of a fork error is commonly forgotten as well. I think that a good way to handle this might be to have a `to_str()` or `fmt::Default` implementation for `ProcessExit` which becomes one of:

* `error code: 100`
* `signal: 10`
* `exec error: failed to allocate memory`

And then error messages like this could be `format!("failure produced {}", status)`