Resolve "branch is not deleted during merge request if "delete source branch" is checked"

What does this MR do and why?

When deleting a branch, we require a lock on the gitaly level, and if we attempt to delete two branches at the same time we may fail to get the lock resulting in the branch not being deleted.

This approach raises an exception on the job, and allows it to retry through the sidekiq retry mechanism.

To Test:

1. Ensure that deleting of source branch works as expected when merging an MR

Related to #27347 (closed)