DocumentsUI: Add an error screen to DirectoryFragment.

- Reorganize the directory fragment.
- Repurpose the "empty" view to hold a message and a button.
- Message is set to the "No items" message if a directory is empty.
- Message is set to the error message if a query error occurs.
- Don't close DocumentsUI when a query error occurs.

Change-Id: I4e1e96f23040606b410ac746252dcb0ab9286f04
4 files changed