Split branch settings LMS 2916

https://edx-wiki.atlassian.net/browse/LMS-2916
fwiw, idk what's absolutely right, but I've started to follow the same indenting on imports as in other code:
```
    from xmodule.modulestore.exceptions import (
        ItemNotFoundError, VersionConflictError, DuplicateItemError, DuplicateCourseError,
        InsufficientSpecificationError,
    )
```