snapshots and snapshot_metadata directories get deleted when you delete all snapshots

[jfischer]

To reproduce:

• created two snapshots
• deleted both snapshots
• tried creating another snapshot. Got the error:

Traceback (most recent call last):
  File "/Users/jfischer/anaconda3/envs/dws-test-environment/lib/python3.6/site-packages/ipykernel/comm/manager.py", line 111, in comm_msg
    comm.handle_msg(msg)
  File "/Users/jfischer/anaconda3/envs/dws-test-environment/lib/python3.6/site-packages/ipykernel/comm/comm.py", line 161, in handle_msg
    self._msg_callback(msg)
  File "/Users/jfischer/code/data-workspaces-core/dataworkspaces/kits/jupyter.py", line 363, in _recv
    message=self._snapshot_args.message)
  File "/Users/jfischer/code/data-workspaces-core/dataworkspaces/api.py", line 103, in take_snapshot
    return snapshot_command(workspace, tag=tag, message=message)
  File "/Users/jfischer/code/data-workspaces-core/dataworkspaces/commands/snapshot.py", line 68, in snapshot_command
    existing_tag_md = mixin.get_snapshot_by_tag(tag)  # type: Optional[SnapshotMetadata]
  File "/Users/jfischer/code/data-workspaces-core/dataworkspaces/backends/git.py", line 445, in get_snapshot_by_tag
    for fname in os.listdir(md_dir):
FileNotFoundError: [Errno 2] No such file or directory: '/Users/jfischer/code/demo-for-talk/demo/.dataworkspace/snapshot_metadata'

Fix is to either always ensure both directories are present before taking a snapshot or creating a placeholder file when the workspace is created.

[jfischer]

dws report history will also fail in this situation