[#180208] more careful _abort_write_group for RepositoryPackCollection

[John Arbash Meinel]

Martin Pool wrote:
> In the bug report <https://bugs.edge.launchpad.net/bzr/+bug/180208> it
> seems that sometimes there is no new_pack when the repository is in a
> write group.  Usually there should be, but if an error occurs at an
> unexpected time it's possible.
> 

You forgot "[MERGE]" but BB:approve anyway.

John
=:->