don't fail migration if account isn't found

it'll just mark the attachment as broken

Change-Id: Ifefb66178d905505b2658b695f148c4f6895c7f6
Reviewed-on: https://gerrit.instructure.com/31170
Reviewed-by: Rob Orton <rob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>