Incorrectly formatted commencement date should show as an error in preview, not throw an error

[sentry-io[bot]]

The errors dict at this point does include something for commencement_date, but we should guard against it not existing (or ensure that it's None) elsewhere so that the error displays rather than a Server 500 error being thrown.

Sentry Issue: INDIGO-LAWSAFRICA-TK

AttributeError: 'SpreadsheetRow' object has no attribute 'commencement_date'
(9 additional frame(s) were not displayed)
...
  File "indigo_app/views/works.py", line 933, in form_valid
    works = self.bulk_creator.create_works(table, dry_run, workflow)
  File "indigo/bulk_creator.py", line 251, in create_works
    self.works.append(self.create_work(row, idx))
  File "indigo_aa/plugins.py", line 51, in create_work
    row = super().create_work(row, idx)
  File "indigo/bulk_creator.py", line 292, in create_work
    row = self.validate_row(row)
  File "indigo/bulk_creator.py", line 409, in validate_row
    elif row.commenced and not row.commencement_date and not row.commenced_on_date: