Correct handling of non-existent stores in open is unclear

[mwatts15]

The doc-comment for Store.open indicates the following (my emphasis):

If create is True a store will be created if it does not already exist. If create is False and a store does not already exist an exception is raised

However, an exception is not raised by the sleepycat store, the only store in the standard library for which the create flag is meaningful. For sleepycat, in the case that create=False and no store already exists it returns rdflib.store.NO_STORE. In addition in the same doc comment it's indicated that NO_STORE is a valid return value from Store.open, but there's no indication of when that should be returned, nor is there any code which expects NO_STORE (or any of the related constants) by name.

It should be made clear what the appropriate action is for a store when create is False, but the indicated store does not exist.