`close_milestone` action should be more forgiving when a milestone is already closed

[iangmaia]

In the CloseMilestoneAction, we first fetch the milestone to be able to close it, throwing an error if the milestone is nil:

milestone = github_helper.get_milestone(repository, milestone_title)
UI.user_error!("Milestone #{milestone_title} not found.") if milestone.nil?

The problem with that is a situation seen a couple of times during release management: closed milestones aren't returned by github_helper.get_milestone.

We should either stop raising this error completely or find a way in the API to get a milestone even when closed, so that at least we would avoid raising the error in such cases and the action becomes a no-op.