Fix Broken Logic in VerticalLayout#getCurrentItemIndex

The `VerticalLayout#getCurrentItemIndex` method will throw if no
item intersects the viewport center. In such cases, return the index
of the item that is nearest the viewport center.

Unit Tests
-------------

- New test for asserting proper handling of error case.

How To +10/QA
-------------

```bash
# skip the following steps if you've already initialized the project
$ git clone git@github.com:WebFilings/wf-uicomponents.git
$ cd wf-uicomponents

# do not skip these!
$ git fetch && 
git checkout fix_broken_current_item_index && 
./init.sh && 
grunt qa
```

- Should see no console errors.

@lancefisher-wf 
@robbecker-wf 
@patkujawa-wf