if update of dandiset fails, reset --hard to the prior state

[yarikoptic]

Relates to #18 -- ATM I ended up with the following history

commit 93c8ea7066e66600952e619243608df0be7dbd76 (HEAD -> master, github/master)
Author: DANDI Meta-user <[email protected]>
Date:   Wed Nov 4 13:12:09 2020 -0500

    Ran backups2datalad.py
    
    3 files deleted

 .dandi/assets.json                                                          | 171 -----------------------------------------------------------------------------------------------------------------------------------------------
 sub-YutaMouse23/sub-YutaMouse23_ses-YutaMouse23-140804_behavior+ecephys.nwb |   1 -
 sub-YutaMouse23/sub-YutaMouse23_ses-YutaMouse23-140805_behavior+ecephys.nwb |   1 -
 sub-YutaMouse23/sub-YutaMouse23_ses-YutaMouse23-140806_behavior+ecephys.nwb |   1 -
 4 files changed, 174 deletions(-)

so we got asset.json removed... AFAIK shouldn't happen. I think the best strategy would be to just revert back completely to the state before even attempting to update