fix: handle CourseOverview.DoesNotExist exception in optout creation

[Akanshu-2u]

Description:

User retirement failed when CourseOverview records were missing, causing CourseOverview.DoesNotExist exceptions in bulk email signal handler.

Solution:

Added exception handling in force_optout_all signal handler:

• Added CourseOverview import
• Wrapped Optout.objects.get_or_create() in try/except block
• Log warning and skip enrollment when CourseOverview is missing

Private JIRA Link:

BOMS-370