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
Small nit--the error you check for can only come from truncindex, which is only about IDs, not names.  So the error message should focus on ID uniqueness--if duplicates were found, then the user did not use enough ID information to specify only one container.

I think it should be something like "ID not specific enough. Please retry with a longer short ID or use the full ID."  Reference: git error on non-unique: `error: short SHA1 cee157 is ambiguous.` (see http://stackoverflow.com/questions/23492381/does-git-warn-me-if-a-shorthand-commit-id-can-refer-to-2-different-commits)

As an aside, to me this is only a short-term bandaid.  IMO `Get` will have to be refactored to return an error as this (and other future error paths) must be able to get this information back to the caller properly--whether via cli or API response codes.  I am planning on working on that PR, but have been sucked into other work and now at a conference all week this week...so, delayed :)