Show branches stacked on a branch when trying to delete

Launchpad does not let you delete a branch that other branches stack on. The existence of a stacked-on branch is vital for the functionality of all the branches stacked on it.

However, sometimes there are good reasons to delete a stacked-on branch. (e.g. https://answers.edge.launchpad.net/launchpad-code/+question/72999). In these cases, we should provide a list of all stacked branches to the deleting user, so they can make decisions about how best to proceed.

At the moment, we simply give the cryptic message:
    "This branch cannot be deleted as it has 1 branch sharing revisions."

which doesn't really help.