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

Reference PR:

• fix: handle CourseOverview.DoesNotExist exception in optout creation edx/edx-platform#77

[robrap]

@Akanshu-2u:

1. Is this a cherry-pick of something we already landed on our fork? If so, can you link to that approved PR in the description?
2. The process for you to work against an openedx repo is to create a personal fork and create the PR there. See our forking docs for details. I'm going to close this PR. Thank you.

[robrap]

This is a cherry-pick of a bug fix that was originally reviewed in edx#77 and is in production on edx.org.