a8n: Gracefully handle changeset deletion in code hosts

Created by: tsenart

Some code hosts allow changesets to be deleted, others don't. For instance, GitHub pull requests can't be deleted (only closed or merged), but Bitbucket's PRs can.

To be reliable in the face of upstream deletions of changesets, the a8n.Syncer must be changed to handle this scenario gracefully.

My intuition is that if the changeset is deleted on the code host, we should delete it on Sourcegraph, as we do for repositories.

But that may be weird if we were the ones creating the changeset upstream and someone deleted manually. @sourcegraph/automation, @sqs: Thoughts on the desirable user experience here?