handle trying to delete folders with content gracefully

fixes: CNVS-15820

test plan:
in newfiles,
try to delete a folder that is not empty.
it should confirm that everything in the folder will
be deleted too.  and then it should work.

Change-Id: I112217486643ca0e73734b417b8648f8e1f4ed0f
Reviewed-on: https://gerrit.instructure.com/42143
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dan Minkevitch <dan@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>