Make split mongo read-only API consistent with other modulestores.

https://edx-wiki.atlassian.net/browse/STUD-617

Some details:

1. modulestore().get_containing_courses was only used in one place, and wasn't actually needed there. I deleted it.
2. get_errored_courses only makes sense for XML modulestore (it wasn't a method of the base class) and was only used in one place. I made that place check if the modulestore has the method.  Deleted dummy implementations from the other modulestores.
3. mongo's get_course_for_item method is just a helper function internal to that modulestore. Renamed it to make it more obvious, and deleted dummy implementation in split.
4. Had to add arguments to some of the split methods to make the API consistent. Documented when these arguments are not used.