Detect unversioned buckets

[drernie]

Quilt implicitly assumes buckets are versioned, so that we can get an immutable reference to any particular object.

However, there is no trivial API for detecting whether versioning is turned on, and it can get complicated if versioning has been turned on and off.

We may need some kind of test file we can read and overwrite inside the ".quilt" folder (if we have read access), to determine whether versioning is fully enabled.