Log when truncindex.Get returns >1 container

When the user is not using the full has to retrieve a container it's
possible that we find conflicts with the ids of other containers.

At the moment it's just failing saying that it can not find a container,
but it doesn't say why. Adding a small log saying that duplicates where
found is going to help the user.

Closes #8098
I feel a bit weird about this "longer short ID" message, and there are typos :-) I suggest:

> "Short ID %s is ambiguous: please retry with more characters or use the full ID."