close dates dialog on invalid save

fixes CNVS-17649

test plan:
  - with DA on
    - edit an assignment and remove the title
    - only assign it to one section and hit save
    - when you hit save the DA dialog opens
    - when you hit save on that, it closes and displays
      the no name error
  - with DA off
    - edit an assignment and remove the title
    - only give a date to one section and hit save
    - when you hit save the dates dialog opens
    - when you hit save on that, it closes and displays
      the no name error

Change-Id: I9e8f5900374f1c80428dba1dc6fafee22c6829b6
Reviewed-on: https://gerrit.instructure.com/46366
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>