Handle case where children are sent TERM
Handle case where children are sent TERM

On Heroku, and potentially other platforms, SIGTERM is sent to ALL processes, not just the parent process. This means that by the time the parent worker tries to wait on it's children to shut down, they may not exist. When that happens an `Errno::ECHILD` error is raised as seen in this comment https://github.com/puma/puma/pull/1808#issuecomment-499874942.

In that situation we don't really care that there's no child, we want to continue the shutdown process in an orderly fashion so we can safely ignore the error and move on.

puma/puma,eileencodes/puma,eileencodes/puma,eileencodes/puma,looker/puma,eileencodes/puma,puma/puma,looker/puma,looker/puma,puma/puma,looker/puma,puma/puma